Question About Audio Research Preamp Specs
dsper OPThis Dsper is BS sorry to say, they either don’t want to do it or have other things in mind, go to a tech that will do it. At least they know that the caps are too small as i first said to you. The person did comment that their latest redesigns addressed this and the output impedance has been reduced. Sure they can’t be put "maybe where the existing ones are, but there are other places they can go. (PS: you can even add (parallel) more of the same type to make what’s there larger no need to get rid of the existing ones) Here in this shot circled in green is the output rca’s of your preamp. Circled in red right next to them is 2 massive amount of spaces that you can put more new caps than you need, take it to someone who knows. https://ibb.co/r4LBDcc Price should be around $40 for two good quality caps, and about a hour labour from what I see. This is a great preamp just needs those larger caps so it can show it’s greatness to you with your amp. Cheers George |
Hi All, For those that might care, I followed up with Conrad Johnson regarding adjusting the output capacitors in my 17LS2 preamp. The technical person I spoke to understood immediately what I was requesting. Unfortunately, they explained that there is no room under the hood to replace the existing capacitors with larger ones. The person did comment that their latest redesigns addressed this and the output impedance has been reduced. I took a look myself and it would seem to my unpracticed eye that there is no space. The inside is chock full of large white "CJ" capacitors already! In the meantime, I will explore different 6922 tubes that might help to mask the LF roll-off. Not sure if that makes any sense... Otherwise, it seems like I may want to replace the 17LS2. Thanks for listening, Dsper |
I do not understand technically why a particular preamp may have a greater capability to drive a low impedance amplifier.No need- I can explain it. There are two factors, output impedance and low frequency cutoff. They are related. First is output impedance and generally to avoid distortion, the amplifier should have an input impedance 10x greater than the output impedance of the preamp. That's easy enough- any preamp mentioned so far does that. The second bit- low frequency cutoff- is a measure of the value of the output coupling capacitor of the preamp vs the input impedance of the amp. The two together form a timing constant. Here's the formula: F = 1,000,000 / C x R x2Pi F is frequency, C is microfarads and R is resistance. Normally the formula looks a bit different (1 divided by the other factors) but since microfarads is a convenient capacitive value I adjusted it. So if we have a 10uf capacitor driving a 10K load at the input of an amplifier, plugging in the values we see that: 1.59 = 1,000,000/ 10 x 10,000 x 6.28 IOW it will be 3dB down at 1.59Hz. The problem is that a 10uf cap has colorations, even if its the best Teflon cap money can buy. But otherwise this cutoff frequency is good. But a preamp manufacturer has to weigh options and one of them might be that they want it to sound better with their own amps. So they might limit the value of the coupling capacitor- and thus increase the cutoff frequency into lower impedances. IOW they sacrifice bass response and impact for greater transparency. But if their ideal amp has a high input impedance they might not be sacrificing any bass at all. Now if you can find a review of a preamp that graphs its output impedance vs frequency, you can see the effect of the coupling capacitor- the output impedance rises as you approach the cutoff (-3dB) point. This shows up in a lot of tube preamps so you can see that many manufacturers regard 10K as perhaps not worth sacrificing the transparency they get with a smaller coupling cap. Because our preamp has a direct-coupled output, our output impedance curve is identical to the frequency response curve. With capacitor coupled preamps this isn't the case. One further thing of note- its a good design practice to set the -3dB point at to at least 1/10th the lowest frequency you want to play, so 2Hz if you want to be good to 20Hz. This insures that there will be no phase shift at 20Hz, which gives you better bass impact. So this is part of the issue- getting that solid bass all the way down. You need that margin in order to avoid artifacts caused by phase shift. IOW if you want proper bass at 20Hz, your preamp should be able to go to flat down to 2Hz while driving the input of your amplifier. |
Why would CJ not have done this in the first place? Maybe a cost/benefits decision considering how relatively few amps have 10K ohm impedance? It wasn't common to have such low 10k input impedance poweramps in the past, as the loosely based standard was 47k or 50k and many especially tube were 100k. Ever since Class-D has reared it's ugly head has 10kohm become more apparent, which means it's not suited to many tube preamps and also many passives. I believe there should be one standard and that's 100kohm then anything can drive it, hell Rogue Audio use 1megohm on their poweramps and they don't have any problems at all. Cheers George |
Atmosphere, " But obviously the Aesthetix faces the same issue of what is the lowest impedance it can actually drive actually; this information does not show up in their specs or operating manual." I do not understand technically why a particular preamp may have a greater capability to drive a low impedance amplifier. While, I try to follow these types of discussions, something basic is lacking in my current understanding as well as how it impacts how it sounds.. Could you perhaps elaborate further or direct me to another thread where I am sure that this has been covered before? Thanks for listening! Dsper |
georgehifi, "Great idea, tell them you want the "uF" of coupling cap increased so you can have "little or no" rolloff above 20hz into your amps 10kohm input impedance." georgehifi, You certainly have given me some discussion points with CJ and what is possible to accomplish. Thank you! Why would CJ not have done this in the first place? Maybe a cost/benefits decision considering how relatively few amps have 10K ohm impedance? Or could this possibly change/degrade the overall sound of the preamp? I assume my last question may illustrate how little I know about this technical stuff so any additional comments would be greatly appreciated! Thanks for listening, Dsper |
Take that with a grain of salt. I do prefer direct coupling (no cap), best cap is no cap mentality.This is why we direct couple... this limitation is eliminated. In the photo of the Aesthetix its not clear what the actual value of the coupling capacitors is. But obviously the Aesthetix faces the same issue of what is the lowest impedance it can actually drive actually; this information does not show up in their specs or operating manual. |
Maybe I should call CJ and ask them if they can adjust the caps on the 17LS2. Great idea, tell them you want the "uF" of coupling cap increased so you can have "little or no" rolloff above 20hz into your amps 10kohm input impedance. This is around $20 for the caps, and a 1hr labour. My calculation for 17ls2’s new coupling caps into the 10kohm of the amp. Any/All are fine. 4.7uf will be -3db at 3hz 6.8uf will be -3db at 2.3hz 10uf will be -3db at 1.6hz Cheers George |
The reason ARC didn't do this initially was simply because the larger you make a coupling cap the more coloration no matter what type of construction.Take that with a grain of salt. I do prefer direct coupling (no cap), best cap is no cap mentality. But dsper OP one of the best tube preamps hailed even by many tube and solid state preamp manufactures as being the best tube preamp ever made is the Aesthetix Calypso preamp, and it has massive output coupling caps, as shown in this pic circled in red. https://ibb.co/4Jb0bSV Cheers George |
1: If you own the AR pre, tell me the size (uF) of the output coupling cap, (it will be the big ones near each output) and what the poweramp input impedance is you want to use it with, and i’ll calculate at what low frequency it’s at -3db for you.The reason ARC didn't do this initially was simply because the larger you make a coupling cap the more coloration no matter what type of construction. Larger caps start to get considerably higher inductance so even if you have the best Teflon its still going to be an audible degradation. A secondary reason might also have to do with space. Adding long leads to make a part fit in a place that wasn't designed for it is a good recipe for noise pickup. |
audioconnection That’s more truth than fiction, we have a very good power amp maker here that has 1kohm input, and yes his preamp is 10ohms, so any other preamp sounds like **** into it but his doesn’t. But the 1k input resistor on the amp is socketed and if removed the input becomes 68kohms, but he doesn’t advertise this, and any preamp/tube ss/or passive will drive it then. Cheers George |
dsper As mzkmxcv stated 8 x output to input ratio is even fine. But in this case the AR preamp would be capacitor coupled output, and the roll off in the low frequency could be too high if the partnering power amp is less than 20kohm input, with the size of the AR’s coupling cap. And so you may miss out on the low frequencies. There are two way this can be fixed, don't get fooled into needing another preamp. dsper 1: If you own the AR pre, tell me the size (uF) of the output coupling cap, (it will be the big ones near each output) and what the poweramp input impedance is you want to use it with, and i’ll calculate at what low frequency it’s at -3db for you. It’s a simple to correct if too small, with a $20 larger coupling cap, don’t go buying another preamp because of it. 2: Or you can go the other way and get the input impedance resistor (20cents) raised to 50kohm on the poweramp, if it’s fet input no problem, even a 100kohm Cheers George |
What does the "20K ohms minimum load" tell me?Exactly what it says. Most ARC preamps are not recommended for amps with anything less than 30K but this one is good to 20Kohms. Your amps though have a 10K input impedance, which essentially means that the combination will be bass shy. As others have pointed out, the output impedance of the LS28 **seems** sufficiently low, but that spec is done at 1KHz. At 20Hz, its output impedance is much higher owing to the size of the coupling caps at the output of the preamp. As a result, the 10K load of your amps will result in a bass rolloff. So ARC is essentially telling you not to use this preamp with your amps. We developed a direct-coupled output on our preamps so loads like the McCormick are no worries. At any rate, if you want to use a balanced tube preamp with your McCormick, ARC is not an option if you want it to sound right. |
I’d recommend against it. I have an LS-28 and a Rogue Stereo 100 which has 200k input impedance and this combination works well. However I once tried to run a subwoofer with a 10k line level input impedance in parallel (causing the LS-28 to effectively see about 9.5k) and the neither the LS-28 nor the Rogue liked it a bit. Sound was distorted and the power tubes glowed red. The ARC recommendation of a minimum 20k load appears to be there for a reason. |
You're reading it right and your 10 to 15 x is generally accepted. The world will not stop rotating if you use a 10k load. I think where this goes is that the preamp may not meet full specs if you feed it into a 10k load. Output voltage may be compromised, THD may be a little higher and low frequency rolloff may be earlier. It will probably be fine and not hurt the preamp but may not sound the same in your system as in another one. The Sterophile reviews for most of the ARC preamps I've seen seem to recommend a minimum of 10K in the test section, biggest concern being low frequency roll off. I've been feeding ARC preamps into 15K for years and they all sounded fine (Ref 2, LS26, LS 25). |
