1.2k Output Impedance to 10k Input Impedance

I spoke to Jenson and was advised that the most appropriate unit for my needs would be the PC-2. it is a 4:1 ratio and therefore would satisfy the needs of both the preamp and poweramp - the preamp seeing a load of 34k ohms and the power amp 225 ohms.... The PI model that you mention would result in a 23.5k ohm load on the preamp and 4.65k ohms on the amp, which if i'm understanding correctly, is less than ideal (ratio of 5:1)

It's not quite that simple, although the conclusion is essentially correct.

The input and output impedance numbers shown in the Jensen datasheets are based on specific source and load impedances ("test circuit 1" shown at the bottom of page 2 of the datasheets), and will vary considerably depending on the output impedance of the preamp and the input impedance of the amp. In this case the balanced input impedance of your amp happens to correspond to the test conditions shown in the datasheet for the PI model, so the 23.5K figure is applicable. But the output impedance of your preamp is much higher than for the test condition shown in the datasheet, so the source impedance seen by the power amp will be considerably higher than 4.65K.

It is important to note, though, that the key concern is not the relation between those two numbers, but the relation between the load impedance seen by the preamp (23.5K in this case), and the actual output impedance of the preamp, at the frequency for which it is highest. See the last paragraph below for further theoretical explanation.

The numbers for the PC-2 (you would want the PC-2RX specifically, assuming that you want to use the XLR inputs of the amp) are based on source and load impedances that differ even more widely from those of your components, and also differ widely from the test conditions of the PI model. That said, though, the PC-2 does appear to be much better suited to your purposes, as does Steve McCormack's Interocitor.

Keep in mind, however, that a volume reduction goes hand-in-hand with the impedance transformation you are trying to achieve. That is specified as 12 db for Steve's unit, and typically 13.6 db for the PC-2. If without the transformer you would be using the volume control in the upper third or so of its range (roughly speaking) that would be a concern. On the other hand, if (as is common these days) you have too much overall gain in your system, and use the volume control in the lower part of its range, that would be beneficial.

The theory involved in all of this is that for an ideal transformer (no practical transformer is ideal, "ideal" meaning among many other things that the DC resistance of the windings is insignificant) the ratio of output voltage to input voltage corresponds to the ratio of the number of turns in the secondary (output) winding to the number of turns in the primary (input winding), while the impedance transformation corresponds to the square of that ratio. It appears that both the PC-2 and the Interocitor have turns ratios of roughly 4 to 1. So the power amp would see an input voltage roughly equal to 1/4 of what the preamp is putting out, which corresponds to a 12 db reduction. The preamp would see a load impedance that is roughly 16 times as great as the input impedance of the power amp. The power amp would see a source impedance that is roughly 1/16th of the output impedance of the preamp. I use the term "roughly" to take into account DC resistances of the windings, various other causes of "insertion loss," variation of the actual turns ratios from the 4:1 nominal value, etc.

Best regards,
-- Al

Let us know what happens.....thanks

@ Grannyring - yes, thanks for the email. i replied, though it seems as if my note might not have made it to you. i spoke to Jenson and was advised that the most appropriate unit for my needs would be the PC-2. it is a 4:1 ratio and therefore would satisfy the needs of both the preamp and poweramp - the preamp seeing a load of 34k ohms and the power amp 225 ohms. Specs are here: http://www.jensen-transformers.com/datashts/pc2xr.pdf

the PI model that you mention would result in a 23.5k ohm load on the preamp and 4.65k ohms on the amp, which if i'm understanding correctly, is less than ideal (ratio of 5:1). Specs are here: http://www.jensen-transformers.com/datashts/pi2xx.pdf

all that said, it's entirely possible i was misinformed by the tech at jensen - he did several times correct himself through the course of the conversation - or i simply misunderstood him.

the SMc Audio transformer is another very interesting option, similar i think, to the PC-2, though designed specifically for this application.

I have emailed you separately. I sold my Jensen and you want the PI model. I will tell you they work and work very well indeed. No loss of fidelity at all. You need to purchase a short run of xlr ic's of say 32 inches or less.

My system with this unit sounded better. Call Jensen and they may let you return it if it does not improve things.

Just get a set of Mogami xlr's for $40 and try it. I am confident you will be amazed.

The preamp to amp relationship is the same as the amp-speaker one as far as source-load pairings go. Just as not all amps are created equal when driving speakers, so it is with preamps when driving amps. Nobody knows better than the preamp's designer what your unit's needs are for best performance, so I would heed his advice.

Now, depending on what the voltage output of the pre is and the sensitivity of your amp for full power, (as well as your speaker's efficiency) you might be able to use a step down line transformer to reduce the output z of the pre. If you were to use a transformer with say a 2.25:1 ratio, you'd be dropping the output impedance to 1200/(2.25^2) = 237Ohms, effectively making the amp's input z 42 times higher than the output z of the pre. This is about the same as if you were driving an amp with an input impedance of 50K from your existing RCA outputs. The trade-off here would be that max output voltage would drop 2.25 times bellow the current value, so if for example the max output of your pre is now 2V, it would drop to a maximum of 2/2.25=0.88V. If this is enough drive to push the power amp to full power then you're good.

For line transformers you could look at lundahl, Sowter and Intact Audio to name a few.

I have an Interocitor One aka Flex-Connect. It is made by SMC Audio and it is superb. I have the model that will step down the output because I have a high gain situation. It will accept rca or xlr on the input and will output rca or xlr, your choice. What's great about this? It can convert rca to xlr or xlr to rca and the amp sees a friendly impedance load. It works like a charm. I love it!!!

I use a Jenson transformer to very good effect. Works like a charm with no negative effects on sound. Only $200 new!

Hi Srosenberg,

The information you've been given here is good, but it still leaves the question of how best to deal with the issue. I understand that this is not the place for advertisements, so let me simply say that I have developed a good solution for exactly this problem. It is functionally similar to the Jensen DM2-2RX but has been designed for use in high-performance audio systems. Please contact me if you would like more information.

Best regards,

Steve McCormack
SMc Audio

In many cases trebble is the most affected area of impedance mismatch. In some cases you may develop lots of clipping even at low volume level and in some cases you'll be a happy listener.

Test your rig with piano music. Solo piano is ideal but you can also use small band or piano duos with violin, guitar etc at different listenable volume settings as you would listen normally.

You may also use some sort of autoformer between components, but with jeopardy to the bottom frequencies.

Hi Scott,

IMO, the impedance matching guideline is properly stated as follows (I'm quoting from myself in another thread):

Ideally the input impedance of the power amp should be at least ten times greater than the output impedance of the preamp, at the frequency within the audible range for which the output impedance of the preamp is highest. That frequency will usually be 20 Hz, as a result of the impedance rise caused at low frequencies by the output coupling capacitor most tube preamps use.

A factor somewhat less than 10x may or may not be acceptable, depending mainly on how the preamp's output impedance varies as a function of frequency, and also on the deep bass extension of the speakers.

The fact that the designer suggests 50K minimum tells me that the preamp's output impedance probably varies considerably as a function of frequency. And that it is much higher at some frequencies, probably including 20 Hz, than the 1.2K nominal figure (which I suspect is at 1 kHz).

The 20K balanced input impedance of the amp probably represents the sum of 10K input impedances on each of the two legs (i.e., on each of the two signal inputs that comprise the balanced signal pair). So using an RCA-to-XLR cable will not help, as you were advised.

Concerning the Jensen transformers, member Mitch2 has an excellent system and has spoken very enthusiastically in past threads about the results he has obtained with them. Atmasphere, on the other hand, has commented that in his experience they do have at least some slight artifacts.

Best regards,
-- Al

sorry for the typo,
"anything less than 50:1 would compromise the performance of the line stage..."

should have read,
"anything less than 50k would compromise the performance of the line stage..."

recommendations seem to vary here - i've read anywhere from 5:1 to 50:1 as a minimum, though most often 10:1 seems to be cited.

while i am just within the 10:1 guideline going RCA into my amp, the preamp manufacturer specifically stated that anything less than 50:1 would compromise the performance of the line stage, the impact increasing with the load on the preamp. hence my thought to convert to XLR.

If it matters, the preamp is a CounterPoint SA-5.1 and the poweramp is a McIntosh Mc252.

I thought the general rule of thumb on impedance matches was that the amp should be atleast 10 times that of the pre. You are just within that envelope, so why not just do it and see (hear)? Might not be perfect, but probably no harm done. Anything patched between them is likely to have a negative audible effect anyway. Others, please correct me if I am wrong here.