"Too much gain"? (Cary SLP05 question)

Hello all,

I thought it would be worthwhile to add to this thread as the unit does have a ton of gain that will severely limit use of the main volume pot. After reading through this thread and others, there has been some good information and some incorrect conclusions. I thought I would relay my experiences here to help future owners resolve the question of what to do with the excessive gain in a pre that they love.

First, let’s be clear what the circuitry is. ALL inputs, xlr (via input tube buffer) or rca get routed to the right and left input trim pots. The pots are a cheap Honeywell plastic potentiometer, 100kOhms. Recommended use for this pot from the Honeywell sheet is in ATVs, joystick controls, and more. Ruggedized for military standards. These Honeywell pots are directly wired to the two plastic potentiometer layers of the Alps 100 kOhm pots, which has a motor control. So you can use either the trim pots or the volume knob to control volume, or both. It doesn’t matter electrically, you just have 2 pots in series.

I have played around with the pots on the Cary, as well as digital attenuation. Re digital, some software driven attenuators are definitely better than others, but it seems like -10- to -15db is the limit where I don’t hear issues in volume matched back to back comparisons. Yeah, I know it shouldn’t be the case mathematically, but something is off. Besides, even -30 or -40 db digital attenuation isn’t enough to give you full use of the volume knob. So that isn’t the answer.

Playing with the pots is interesting, because you will quickly notice with volume matched testing that the balance trim pots are clearly filtering the signals. This shows up with low level information at all frequencies, but it is most easy to see with higher frequencies. Maybe a system runs a bit bright? Well ok, but the full spectrum loss of clarity and detail is not a favorable trade. Buy tubes that are rolled off on top, *cough* I meant warm sounding.

Playing with the Alps volume pot gives similar results, but with less severity. The channel imbalance near max attenuation is a problem though, as is the high frequency filtering which makes the bass seem exaggerated.

So what are the solutions? I guess you have to decide what you want out of this pre. Do you use the headphones a lot? The input trim will likely be important unless your tubes are perfectly balanced. Input trims are useful for troubleshooting, but that is limited case scenario when it is also easy to just pull a cable. Only use the volume knob and you just want it to work better?

If you want it all I would look at replacing the volume and the trim pots with high quality stepped attenuators. Goldpoint,Khozmo, or others are good examples. Yes they are expensive. But your $6-8k amp deserves better than 3$ and 12$ pots. I put in a 24 step mono 100k pots in place of the trim as I don’t use headphones. If you do, maybe see if 47 step will fit.. First off, night and day improvement in clarity at all frequencies, you will sit up in your chair. Second, with high quality pots you will notice that the Alps isn’t exactly bad. If you do most of the attenuation with your stepped pots, then you can run the Alps from 12o’clock to full on with good results. In messing with this configuration over the last month, I would say that the less the Alps is in the chain, the clarity improves but with diminishing returns. My next step will be to replace the Alps with a 47 step attenuator and call it good. The upper range of the 24 stepped input trim is just a teensy bit limiting in my system.

Motorized volume knob mandatory? Well there are solutions depending on the attenuator you choose, but they are expensive. I would go this route if you plan to keep the pre for as long as you can. I haven’t decided what to do yet, I might see if I can use the existing RC design and fabricate an adaptor for the new attenuator. We’ll see.

Hope this helps. Yes, get a professional to do the work and the usual disclaimers. I have no knowledge of EE or circuit designs, these conclusions are my own experience and you may get different results. Make modifications at your own risk.

Find this an interesting thread as I’m still looking at an SLP-05 as a second preamp.

Enjoythemusic dot com published a review of the Rothwells that say they do affect sound quality. Never used them myself but seeing here they seem quite popular.

@jaybe 

Having lived with the ins and outs of the SLP05 I am still enthralled with the sound quality of this preamp.  

The extra gain can be too much but there are two ways to satisfactorily work with it.  BTW the higher gain is a virtue as the resulting dynamics make for a superior sound stage and listening experience.   Also the rothwell 10db attenuators did not work for me either. 

The easiest way to deal with it is to use the single ended outputs which gives you 17db which is perfectly suitable for most amplifiers and just two db more than most preamps.  This is my current setup feeding my new gen McIntosh MC312 power amp and the sound is both dynamic and pristine. 

The second way is to use an amplifier with lower than usual gain and use the balanced outputs higher 24 db gain. I used it with the Pass Labs XA25 that has 20db gain with stunning results.  Also the Benchmark AHB2, many of the First Watt amps as well as Bryston models that have a two way gain switch one being 22 db.  

Its incredible what this preamp can do in stock form, even better with NOS tubes and rectfier.   Dont let the higher gain scare you away.  

I haven’t tried the Rothwells, so I cannot comment on those directly.

In regards to attenuators, there are some things that should definitely be pointed out.  First off, any attenuator that isn’t matched to output and impedance runs the risk of increasing roll off in the upper or lower frequencies.  Because attenuation works by dumping voltage to ground, and allowing the remaining voltage to pass to the input circuitry, current is also passing to ground.  I tried some basic 600ohm T style first because the case was reusable and I could always try different topologies, buy as many resistors as I wanted to find the sweet spot.  The 600 ohms dumped so much current the tubes were unable to drive any low frequencies.  Keep in mind that the 400 ohm output impedance of the Cary rises to 4000+ as the signal drops to ~20 Hz, as measured by JA in the Stereophile review.  This unit runs out of juice quickly unless you have a high input impedance amp.  50k+ would be my recommendation.   Note that OTHER highly regarded tube preamp makers state they can increase the cost with beefier output capacitors to match equipment.

I was able to find a resistor match that did measure tonally the same. However, and these are my listening impressions only and should not be regarded as a claim to anything but, I found after considerable testing that the XLR inputs and outputs of the Cary sound different (edit: compared to RCA).  With 6 identical tubes, the XLR pathway loses warmth, bloom, depth, microdynamics and more.  Some say adjust with the input trim.  That is BS, they are directly wired to the Alps pot.  Putting 2 cheap plastic trim pots in series serves no purpose except to allow balancing for mismatched tubes in the headphone stage where it will be most noticeable.   Regardless, both the input and output XLR circuitry both have the same character (edit, compared to RCA).

I did ask Cary about this issue and if they would recommend a local service place to diagnose the issue, and they didn’t reply.  Commonly held belief is that properly made RCA and XLR circuits should sound identical.  Indeed, in my other gear they do.  

Having said all that, running RCA with only 4 tubes (leaving 4 empty), replacing the Alps pot with Kozmo, Goldpoint, or other high quality attentuators and removing the input trim pots will leave you with the potential for glorious sound, as long as you buy exceptional tubes.  I did the work myself, but it will cost you around $1k if you get a tech to do it.  

The gain issue should not be ignored.  Running my XLR 4V outputs through the Cary into a Pass amp (+26 db) required the attenuators to dump 99% of the voltage to ground after the input tubes going by resistance level and L circuit configuration. Many users note minimal volume control issues.    Note also the alps is also the most susceptible to internal RMF signal interference at the bottom and top 1/4 of its range.  Note that with output relays turned off you can clearly hear an input signal at low and high volume, but not at 12o’clock.  So you have to be able to run the unit at 11-2 ish to get best sound.  Gain matching and impedance matching are critical to getting great sound with this unit.  YMMV, and others have reported great satisfaction.  

Cheers

Edited to remove ambiguity, and noted as edits

I found after considerable testing that the XLR inputs and outputs of the Cary sound different (edit: compared to RCA).

@khloebo , my impression is the complete opposite.  I have found that when comparing SQ of the balanced circuit vs the RCA circuit, that after listening to my system using the XLRs the sound stage seems  folded in or collapsed when I use the RCAs.  

Out of curiosity, and not that it would have anything to do with perceptions of balanced vs single ended ins and outs, does your SLP-05 have the Cary Upgrade package?