"Too much gain"? (Cary SLP05 question)

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?  

@immatthewj   Hi there!  Good point for the discussion, no mine does not.  I bought it 2014 as well, it probably has around 7000 hours on it.  

The upgrade package reportedly has a complete capacitor overhaul, which could affect the SQ in a number of ways (especially driving 20k input impedance).  However, I have not read anyone doing a direct comparison between the two, which is not surprising.

Also, and this is just a giant guess as I haven't created a circuit design and asked an EE about it, but if the ultimate upgrade improves the capacitors, AND the circuitry that converts the balanced signals to unbalanced (and vice versa for output) then it absolutely should be better in technical performance and adding input tubes should add tube character. 

One of the reasons why I replaced the input trim pots with Goldpoint attenuators is so that I could figure out some issues I discovered with tonal shifts during digital vs analog attenuation.  Long story short, both the cheap honeywell pots and the Alps act as low pass filters when attenuation is nearing the max.  IE, they remove high frequencies.  Thus sound is perceived to be rich in mids with fat bass.  Using XLR and low volume controls increases this effect.  It can be very pleasing and rich!  However, it comes at the expense of clarity and dynamics.  Listening to a bunch of systems over the last couple years with tubes left me unsatisfied with mine, so I tinkered.  The Goldpoints allow tons of clarity, richness, detail, space and everything else through.  I keep the Alps at max so it is out of the circuit for music, it only gets used now for TV volume.

Having said all that, nothing needs to be done when you love the sound, XLR or RCA.  New purchasers should definitely try the unit before buying or with a full return option.   It is an interesting box that needs the right situation to sound at its best.