Amp preamp impedance matching...can anyone explain?

@mulveling 

Yeah, I guess I drove it off the rails a bit, but no one else seemed to be paying any attention to the original part of the discussion, so I figured no big deal.

@dpop 

Right near the start of that article: "600 Ohms in order to achieve maximum power transfer to the receiving equipment". Power transfer is not a requirement in this application. It's an interesting article and I'll read it, but I also think this is all confusing the main topic here, which concerns impedance matching a preamp to power amp for efficient voltage transfer, not power transfer - all meaningful power is supplied by the power amp reacting to the input voltage.

@mulveling 

You might find this article interesting:

THE 600 OHM AUDIO STANDARD, WHERE DID IT GO?

@dpop 

It is an interesting topic - but it seems likely that studio gear with 600 ohm loading requirements (apparently to damp ringing?) is operating with very different needs (power, bandwidth?) than a high-end 2ch home audio preamp. Here we need only efficient voltage transfer; no power. Ringing apparently not an issue with good line-level output transformers into any power amp load. Again, the VAC Renaissance V is not prescribing a specific load impedance. 

I do remember that a few headphone models present 600 ohm loads, usually from Beyerdynamic or AKG, which are brands that seem a bit more concerned with studio use than other headphone makers. Most dedicated headphone amps (which are typically OTL and "high current" designs) have very low output impedance, and have no problem driving those or any other headphones. I haven't seen a 600 ohm load specified or required for any other piece of home audio gear in many, many years.

Back in the days of tube audio gear in radio stations, almost everything was 600 ohm transformer coupled (inputs and outputs). Some professional broadcast solid state gear was even transformer coupled (not sure why, except for maybe RFI rejection). As you can see in my link above (600 ohm output transformer termination), there is still sometimes on-going discussion on the subject, as some highly sought after vintage pro processing gear has 600 ohm transformer outputs. AKG even made a K240 headphone that was rated at 600 ohms. I’ll have to dig back into some of my old broadcast magazines to refresh my memory on the subject, since it rarely concerns me anymore.

I can’t, because I don’t know what gear that is or its intended use (you mentioned radio broadcast). A 600 ohm load requirement doesn’t sound like anything related to high end home audio, honestly. I can speak to the VAC Renaissance V because I owned one and know its intended application. It is designed to work well with as wide a range of 2ch SS and tube power amps as possible.

@mulveling

OK, that sounds good to me. Maybe you can explain (in better terms) the need for the 600 ohm terminating resistors with older transformer 600 ohm output gear?

Many tube preamps will require a minimum load of 10K or higher, which can become a restriction with some SS amps.

Interesting. I don’t think I’ve ever seen a solid state audio impedance input rating lower than 10K.

It’s absolutely not needed here. In fact, I think it would be a very bad idea to feed a Ren V into 300 ohms - that’s the lowest impedance that it will work “ok” with. I’ve used the Ren V into 100K ohm tube amps no problem. I now use the VAC Master, same circuit, same specs, same deal (just better sound quality from premium parts and PSU). Also works great into a SS amp. The “> 300 ohms” spec is intended as a flexibility boast, not a requirement or limitation. Many tube preamps will require a minimum load of 10K or higher, which can become a restriction with some SS amps.

@mulveling

it specifies a recommended load impedance of greater than 300 ohms

I totally understand the concept of solid state low impedance op amp output feeding a solid state op amp high impedance input. No questions there, but these transformer outputs (if that’s what the Ren MK V has) have confused me in the past. There’s no doubt in my mind about terminating 600 ohm transformer outputs before feeding them to solid state op amp inputs. That’s definitely necessary, but I’m not 100% sure in this situation if a 300 ohm termination is needed. I suppose Renaissance technical support could answer that question.

600 ohm output transformer termination?

@dpop it specifies a recommended load impedance of greater than 300 ohms. That includes loads like 10K, 50K, 500K, etc. This is not like matching taps on power amp transformers, where maximum power transfer is the goal. Here we are only concerned with transferring voltage signal, where Ohm’s law dictates a good transfer. That means you want as low an impedance on the source side (150 ohms being very low for a preamp, especially at 20 Hz), and a “sufficiently high” impedance on the amp side (300 ohms being far lower than typical). It is reasonable to aim for a load impedance 20x (or more) greater than the source impedance to minimize losses caused by voltage division.

@mulveling  @river251 

The Ren V claims an even 150 ohms or less over the entire music bandwidth

That looks to be true, but if you look at the pre-amp specs, the Ren V wants to see a recommended output load of 300 ohms. Most solid state pre-amp's have input ranges of 10K to 47K (using those figures as general examples), which will not provide this load. As the radio broadcast industry segued from transformer outputs, to op amp inputs/outputs, many transformer devices (audio gear) incorporated selectable 600 ohm terminating resistors, so that you could load the transformer output (a perfect example was 600 ohm transformer outputs) if feeding a high impedance input. I think many tube pre-amp owners need to be aware of transformer output loading when feeding a solid state high impedance input. 

Renaissance Mk V Pre Amplifier technical specifications 

Transformer coupled output is an excellent approach, especially for tube preamps which otherwise contend with the noted issues of output caps and rising impedance. The Ren V claims an even 150 ohms or less over the entire music bandwidth, and so should have no problem driving any SS or tube amp. Using transformer coupling for both inputs and outputs also frees up the preamp circuit to use “only” 2 tubes, and handles SE to balanced (and the reverse) conversions “for free”. 

Sorry for reviving this important thread.

In the posts above, we see conversation about output impedance of (tube) preamps that have a coupling capacitor (the fact that, most likely, the output impedance shall be highest at 20Hz).

What about impedance matching for tube preamps like the VAC Renaissance Mk V that have transformer coupled output (and input) stages?

I don't know if this thread is still active (if not I'll have to find another forum).

I'm driving a Quad 303 power amp from the headphone socket of my Yamaha Amp.

From the published specs:

The Quad Power Amp i/p is 0.5V @ 22K ohms impedance

The Yamaha Headphones o/p is 0.51V @ 330 ohms

It looks like the impedance match is ok, but I had to attenuate the signal with 1MΩ resistors which I soldered into the DIN plug (Quad i/p).

What I want to do now is connect to the aux i/p of a Cambridge amp; the impedance is 47KΩ (the signal requirements aren't mentioned in the specs, but I would think it'll be around 15omV). From what I read here this will be a mismatch one option is to solder 47KΩ resistors across the i/p's (sig to scr) in the phono/rca plug; I don't really want to go inside the amp and change the resistors to lower values.

I'll also need to attenuate the signals perhaps with higher values than 1MΩ.

I wonder what the experts think.

Regards:
-Pete

Bifwynne, that is a common problem if you are trying to use a balanced preamp with a subwoofer. Many subs only have SE inputs.

Another solution is that Jensen transformers makes a transformer that is optimized for subwoofers. It has a fairly high impedance input that can be run balanced or SE, and has bandwidth to less than 2 Hz so it does not mess up the bass.

But I think you are lucky you ran into Tom Tutay. He does good work :)

Al, thanks so much for your explanation - I get it now, although coupling capacitors and such are still beyond the scope of understanding for me :)

The only regrettable part in all this is that it seems impossible to determine good impedance matches based on the nominal output impedance spec that the preamp manufacturer will show. Everyone would have to rely on magazines like Stereophile or audio websites to actually review a certain product and show readers the detailed impedance measurements across the whole spectrum in order to do the correct calculation. This is not a problem for popular brands like ARC or Cary, but the lesser known brands might not get a review, and then we audiophiles are taking an educated guess as to whether a preamp is a good match impedance-wise with a power amp.

Al and Ralph, both the ARC Ref 3 and 5 literature recommends that the minimum combined load on the Main outputs is 20K Ohms. My amp is the ARC VS-115 which has an input impedance of 300K Ohms (balanced) -- so no problem there. If you pull my threads, you'll see where I got into a problem was when I asymetrically loaded the 2nd Main output with a 20K Ohm load. Main 1 -- VS 115 (balanced) 300K Ohms; Main 2 --self powered subwoofer (SE) 20K Ohms.

As I mentioned in my old threads, the solution was that Tom Tutay designed and built an impedance buffer device that summed the left and right channels without shorting the Ref 3/5 output Mains and enabled me to load Main 2 (self powered woofer) with a balanced input having 330K Ohms impedance. So my overall preamp output impedance load is 157K Ohms -- well above the recommended minimum.

Result -- tighter and more extended bass. Possibly more open midrange. Per ARC, symetrically loading the Ref 3/5 will also extend tube life -- but what do I know???? FWIW.

Hi Ralph,

Yes, I seem to recall that being true of some of their later preamps and/or phono stages. For the Ref 3, though, the specs shown here, and also in the Stereophile review I linked to earlier, indicate 20K. Although the wording leaves it unclear whether that applies to the balanced or unbalanced outputs, or both.

In any case, given the 30K impedance of the balanced inputs on the Pass, and the measured worst case (20 Hz) output impedance of 1437 ohms for the Ref 3 (which it should be noted is only a bit more than a factor of 2 greater than the midrange output impedance), it would appear that there shouldn't be any frequency or phase response issues. And I'd be surprised if there were any audible distortion issues either, although you can speak to that possibility more knowledgeably than I can.

Best regards,
-- Al

Al, isn't it the case that ARC does not recommend less than a 30K load with their preamps? I know that is true of at least one of the recent Ref series.

Lpw, as I indicated earlier what is important is the preamp's output impedance at the (audible) frequency for which that output impedance is highest. That figure will often occur at 20 Hz, especially in the case of a tube preamp (due to the coupling capacitor that most but not all tube preamps have at their outputs), and in the case of a preamp having an output coupling capacitor will very often be much higher than the specified value. The specified value can be assumed to be based on midrange frequencies, unless explicitly indicated otherwise.

According to Stereophile's measurements:

Ref 3:

The output impedance was also to spec., at 635 ohms balanced and 326 ohms unbalanced in the treble and midrange, but rose to 1437 ohms and 625 ohms, respectively, at 20Hz.

SLP-05:

The SLP 05's output impedance is specified as a usefully low 400 ohms. However, I got significantly higher values for the unbalanced output of 1500–1600 ohms in the midrange and treble, rising to 3400 ohms at 20Hz, with similar if slightly lower figures for the balanced output.

The specs for your X250.5 indicate an input impedance of 30K balanced and 20K unbalanced, which are the same as the input impedance specs for many of the other Pass amps. At some point in the past, though, their unbalanced input impedance may have been spec'd at 15K, lower than it really was and is. See the specs and measurements in Stereophile's review of the XA30.5. That discrepancy perhaps contributed to some misconceptions.

The XP-10 and XP-20 are spec'd as having output impedances of 1K balanced, 150 ohms unbalanced. Presumably balanced connections would be used between all of these particular components.

Based on the foregoing numbers for balanced connection, the Cary's output impedance rise at deep bass frequencies results in it not meeting the ideal minimum ratio of 10 that is applicable if the preamp's maximum output impedance across the audible frequency range is known (30K/approx. 3.4K = 8.8). Although it comes close enough to probably be marginally acceptable in some systems (depending on the deep bass extension of the speakers, for one thing).

The Ref 3 should be no problem, in terms of impedance matching. 30K/1.437K = 21.

Although I couldn't find measurements, it can be presumed that the specified 1K output impedance of the solid state Pass preamps does not rise significantly at 20 Hz (or other) frequencies, so it too should have no problem working into a 30K load.

Regards,
-- Al

I have a Pass X250.5 amp whose input impedance is 22 kOhms. I've been reading about tube preamps, specifically the ARC Ref 3 (output impedance 600 ohms balanced) and Cary slp-05 (output impedance 500 balanced), both of which seem to draw comments that they wouldn't be good matches with Pass amps because of the low input impedance of the Pass amps.

So I went on the Pass website and see that the XP10 and XP20 preamps have output Impedances (for balanced) of 1,000 ohms! Presumably, these two preamps are ideal matches with my X250.5 amp, but if I do the math, how is an XP10/XP20 with 1000 ohms output impedance a good match for the X250.5 when an ARC Ref 3 with 600 ohms output impedance is considered by some to be a bad match? 22000/600=37 for ARC and 22000/1000=22 for Pass XP10/XP20

What am I missing or not understanding?

Lpw

07-24-12: Distortions
Does the voltage gain play into this at all?

No, gain and impedance compatibility are two separate issues, although if the amplifier's input impedance is too low in relation to the preamp's output impedance (which it is not in this case) there will be a very slight reduction in overall gain.

I see that in stock form, prior to the recent modifications, your preamp had a line stage gain of 26 db. That in combination with the amp's 30 db gain, as you realize, is way too high, especially if you are using a digital source having the typical full scale output of 2 volts or so.

I see that you have another thread in progress on amplifier replacement and other possible approaches, and you've already received an intriguing input from Steve McCormack himself!

Regards,
-- Al

Does the voltage gain play into this at all? The amplifier I am using is a McCormack DNA-125, which has 30db of gain. Its input impedance is 100 kOhms. I am using an Audio Research SP8 MkII preamplifier with it which has an output impedence of 1,000 ohms.

I had Audio Research lower the gain in my SP8 recently, but needless to say, the volume through my Vandersteen 3A Signatures still gets loud in a hurry.

I was wondering if someone could please recommend a low gain, solid state Audio Research amplifier that is rated between 100-200 watts per channel? I do not want a tube amplifier, and I do not want to get rid of the wonderful SP8 MkII.

07-19-12: Dracule1
I have a tube DAC that uses a single 6SN7 tube with 1 microfarad coupling caps. I was told it has 8 kohm output impedance.

that is correct. impedance at 20Hz = 1/2*pi*20Hz*1e-6 = 7957.7Ohms, which could be rounded up to 8KOhms. The manuf wasn't lying! ;-)

I wanted to get this integrated SS amp but the input impedance is 10 kohm. So this is a horrible match, so I stayed away.

right again! a horrible match down the in bass region. Your system would have sounded tinny. So, good thing you stayed away.

But I really wanted that SS integrated.

better put a lid on that desire while you have this particular tube DAC! ;-)

Thanks for clearing this up here...

Vineman, since the worst case (20 Hz) output impedance is known, just multiply that by 10 to calculate the minimum desirable amp input impedance.

495 ohms x 10 = 4.95K (minimum)

Nearly all power amps will meet that criterion.

Regards,
-- Al

Reading this as well and I wanted to add my specs from my Marantz PM-KI-Pearl Integrated that I would like to entertain using as a preamp and then adding an amp...tube or ss still not determined.

The testing of this Pearl unit shows "The output impedance from the Preamp Out jacks was 217 ohms at high and middle frequencies, rising to 495 ohms at 20Hz."

Based on this information, does one still look for the above in the input impedance of 47k or higher?

I have a tube DAC that uses a single 6SN7 tube with 1 microfarad coupling caps. I was told it has 8 kohm output impedance. I wanted to get this integrated SS amp but the input impedance is 10 kohm. So this is a horrible match, so I stayed away. My tube amp with passive stepped attenuator at the input of the amp has 100 kohm impedance. This DAC/amp combo sounds dynamic as hell with deep bass extension. But I really wanted that SS integrated.

I am really learning a lot from you guys. Thanks.

I was just playing with the thought on the different types of power-amps that my preamp of 20 ohms output, can be connected to! Seems like it will be alright. ;)

As a simple rule of thumb most any transistor preamp can drive any tube amp.

Its not always the other way 'round as pointed out above. (FWIW we get around the problem by direct-coupling and using paralleled tube sections to obtain a low output impedance that can drive any amp around.)

But if there are coupling caps its a different ballgame. For example, ARC recommends no less than 30K as the input impedance of the amplifier with which the Ref 5 is used.

10:1 is a safe minimum value, but you can go 100:1 and its fine. So in the ARC example above, standard engineering practice suggests that the output impedance of the Ref 5 is 3K ohms. However they are fairly conservative and are showing 600 ohms on their website, although that might be at 1KHz. IMO the 20Hz output impedance tells the real story.

The lower the output impedance, the more the line section can control the interconnect- by that I mean the less sonic attributes the cable will have. So the lower you can get the output impedance, the better (all other things being equal...).

Is there a limit to how high these ratios or the power-in should go, before it sounds bad?

No, not that I can envision.

Regards,
-- Al

We have been talking ratios of pre-out to power-in as 1:10, 1:50, 1:75 or absolute numbers as 47,000 ohm.

Is there a limit to how high these ratios or the power-in should go, before it sounds bad?

I am a layman asking this... Thank you

I would like to add another factor to the discussion.

The ratio of 10/1, while ballpark, may not be the only factor if the solid state device uses operational amplifiers in the output stage. Many opamps have output impedances well below 1K, but if you try to drive a 10K load with these (particularly at voltages over 1 V RMS), you can often get flat, lifeless sound performance. Of course, the results also depend on the type of opamp used. Some are better than others.

For opamp driven output stages I suggest an imput impedance of 47K or higher, with output voltages below 1-2V if possible, irrespective of output impedance.

Even with an ideal pre-amp to amp impedance match the B&K ST 140's sonic signature (as is the c-j's too) is a bit soft in deep bass, compounding it with a less than perfect impedance match might not be the best choice. I wouldn't recommend a low impedance speaker (<4 Ohms) with the B&K ST 140 either. Which is not to say that in an appropriate system the B&K ST 140 is not a good amp, as it certainly is, especially for the money. Keep in mind that the B&K ST 140 is getting a bit long in tooth and B&K is no longer in business. Though I would imagine that having the B&K ST 140 serviced by an outside source wouldn't be too difficult should the need arise.

Thanks for chiming in, Bombaywalla, and saving me some time :-)

I found some information specific to the CJ PV-5, though. The schematic and parts list can be found here. It can be seen that the output coupling capacitor is two capacitors in parallel, totalling 2.35 uF. At 20 Hz, that corresponds to an impedance of 3388 ohms, based on 1/(2piFC). (That figure will be vastly lower at mid-range and treble frequencies, consistent with the 200 ohm nominal output impedance). It is most likely safe to assume that the overall output impedance at 20 Hz will be just a little higher than the impedance presented by the capacitors at that frequency, so let's call it 3500 ohms.

22K/3.5K and 24K/3.5K are ratios that are between 6 and 7, lower than the ideal of at least 10. So there may be a slight impact on the deepest bass frequencies, which may or may not be perceptible depending on the deep bass extension of your speakers, and on room acoustics. If your speakers have good deep bass extension, my instinct would be to play it safe and avoid the B&K and the Aragon, although it's a close enough call to be debatable.

Regards,
-- Al

07-11-12: River251
Thanks Almarg. What sort of frequency response irregularities?

If I may be allowed to chime in - like Almarg wrote "Many tube preamps use a coupling capacitor at their outputs, which can cause their output impedance to be much higher at deep bass frequencies than at higher frequencies". So, the preamp output impedance is usually not a constant. It's often lower in the mid/hi freq & higher at bass freq. When higher in the bass freq, you *might* see a loss of signal power from pre to power + the loss of control by the power amp of the bass region. The listener usually perceives this as flabby/bloated bass.
I know that a friend of mine experienced this with a highly regarded preamp that had 600 Ohms output impedance while his power amp had 10K input impedance. It was a bad match & the preamp was sold off immediately.

I have a CJ PV-5 (only thing I found was a post saying 200 ohms output impedence) and Marantz 8B (and Mac MC240 and MC30s but listening to the 8B). I am considering trying a SS amp and considering B&K ST-140 (24Kohm input impedence), Aragon 2004 MkII (22Kohm), and McCormack DNA .5 or 1 (100Kohm).

from my personal experience this should not be an issue. My preamp has 200 Ohms output impedance & my power amp has 10K input impedance. The 2 units mate up very nicely across the entire audio freq range.
FWIW.

Thanks Almarg. What sort of frequency response irregularities?

I have a CJ PV-5 (only thing I found was a post saying 200 ohms output impedence) and Marantz 8B (and Mac MC240 and MC30s but listening to the 8B). I am considering trying a SS amp and considering B&K ST-140 (24Kohm input impedence), Aragon 2004 MkII (22Kohm), and McCormack DNA .5 or 1 (100Kohm).

Regarding:

If, as is often the case, the highest output impedance of the preamp across the audible frequency range is not known, and only a nominal output impedance is specified (perhaps based on a frequency of 1 kHz), I suggest using a ratio of 50 or more, and preferably 75. Many tube preamps, and some solid state preamps, use a coupling capacitor at their outputs, which can cause their output impedance to be much higher at deep bass frequencies than at higher frequencies.

....for the gear I mentioned, do you think I need to worry about the 50:1?

Thanks very much,
Jim

Onhwy61, I think(?) I remember Jeff Rowland espousing such a philosophy. I also seem to remember(?:-)) past Audiogon contributor Sean suggesting the same thing. Some years ago Dave Wilson writing in The Absolute Sound offered a tiered approach to impedance matching.

Onhwy61, that would apply to situations where the component providing the signal is specifically designed to be able to drive 600 ohms, without a significant increase in distortion or a significant impact on bandwidth or frequency response.

Most consumer gear will not meet those requirements. I believe that Atmasphere is a notable exception. According to posts Ralph (Atmasphere) has made in the past, a benefit that would result from loading the balanced outputs of his preamps with a low impedance like 600 ohms is minimization or elimination of cable effects and cable differences.

Best regards,
-- Al

I too have heard the 10x ratio, but are not there some companies that preach a transmission line approach? I have seen references to a 600 ohm standard in older studio equipment.

Ideally the input impedance of the amp should be 10 or more times greater than the output impedance of the preamp, at the frequency for which preamp output impedance is highest. Otherwise audible frequency response irregularities MIGHT result.

If, as is often the case, the highest output impedance of the preamp across the audible frequency range is not known, and only a nominal output impedance is specified (perhaps based on a frequency of 1 kHz), I suggest using a ratio of 50 or more, and preferably 75. Many tube preamps, and some solid state preamps, use a coupling capacitor at their outputs, which can cause their output impedance to be much higher at deep bass frequencies than at higher frequencies.

Impedance incompatibilities are most likely to be encountered when using a tube preamp with a solid state power amp.

If the power amp has an input impedance of around 47K or more, it is unlikely that there will be an issue, even with a tube preamp.

Regards,
-- Al