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Impressions of PS Audio GCPH

I recently obtained a PS Audio GCPH and there are a few observations of the circuitry that surprised me, in the face of the advertised information provided by PS Audio.

First, as some have mentioned in the past, the noise performance limits the actual gain you can use with this device. For example, the 48 dB gain setting is only 48 dB when the front panel gain control is maxed out (full CCW). But if you use full CCW, the noise is intolerable. In my system (Parasound JC2 line/JC1x2 power/B&W 830d) I can use a max gain of 12-1:30 on the GCPH before I can hear audible noise. That is with the input source impedance loading set to 1K (which is typical for midband MM cart impedances). Setting the loading down to 100 Ohms improves noise a bit. The noise was not all hum, but mostly white thermal noise, which means the transformer orientation inside the GCPH is OK. At the volume control setting of 1:30 (12 being straight up), I measure the gain at 40 dB with 3 mV (at 1000 Hz) in, 300 mV out. Considerably under the 48 dB stated.

Checking the other gain settings I also found that the usable gain is about 6-8 dB below the marked settings. I checked the highest gain setting of 66 dB and got about 60dB actual usable gain (.5 mV input, 500 mV out) at the 1:30 volume postition).

I think this is still high enough for most cartridges, except for really low output (150 micro-volt) MCs. Its just that the advertised and marked settings are misleading, particularly if you need the higher gains.

A word about my noise tolerance criteria. I find noise level unacceptable if I can hear anything out of the drivers (with my ear at 6") at my normal listening volume. With my CD playback system (CA 840/Bryston BDA-1) the noise level is undetectable at this same level (and to even much higher gains), so the phono preamp should be able to reproduce this as well.

The other observation I found concerning was that the actual circuitry uses two monolythic IC circuits for the preamp. The device is an Analog Devices SSM2019B pre-amp. I was under the impression that the GCPH used only "fully balanced True Class A circuits through-out" (Ryan Conway, PS audio review on Audio Advisor), meaning discrete Class A circuitry. It is not. The SSM2019B is not differential balanced, and its questionable whether it is Class A biased either. The gain cell modules appear to be output buffers.
dhl93449
swanny76109
382 posts
 
are you running the outputs fully balanced via XLR? Those outputs will give 6 db higher than the single ended outputs.
dhl93449 OP
455 posts
 
Swanny:

No, I was using the single ended outputs. I am not sure about the 6 dB more gain beacuse it depends on how the line pre-amp handles balanced inputs. Uncertain about the JC2, but some pre-amps I know of divide back the 6 dB gain when they convert back to single ended internally, so they match the same levels of other singel ended inputs.

Since my original posting I had a chance to make some measurements on a spectrum analyzer. I thought I might have a defective unit. Although PS Audio does not publish noise specifications, I thought I might compare it's performance with some other products (such as the Cambridge Audio 651, which has some pretty good noise specs).

What I found was the noise performance is pretty good. At the x48 dB gain position, I got a noise level of -105 (200 Hz) to -110 (20 KHz) dBV (ref to 1 V RMS output) at the 1:30 volume positition, or 40 dB actual gain. Moving the front volume control to MAX CW position, gain is x48 dB, and noise increases to -90@200 to -100@20K dBV, which tends to indicate you are picking up a bit more noise than you are gaining in gain. These measurements were made with the internal 1 K source resistance (not shorted), which is more realistic source impedance of a MM cart. These are raw measurements, with no weighting (A or C). The range of about -10 dBV indicates the drop in noise with frequency from about 200 Hz to 20 KHz.

At the 66 dB gain position, I measured noise levels at the 1:30 volume position (actual gain of 60 dB) to be essentially the same as the x48 dB gain setting; -90 to -100 dBV. But increasing the volume to MAX, noise increased to -75 dB (200 Hz) to -90 dB @ 20KHz. Source resistance was 100 ohms. Here you lose a wopping 15 dB SN for an increase in gain of only 6 dB.

So at x66 dB, MAX volume, you get a worse case noise level of -75 dB, un weighted, which is pretty good. Weighting and shorted inputs always improve the numbers somewhat. But -75 db is pretty audible based on my earlier criteria of listening to the drivers directly. The SN in my system is worse than -75 dB because my gain is set for a listening level ref to 300 mV, not 1 VRMS (which is now actually -65 dB ref 300 mV RMS).

So with the GCPH, if you need gain use the main gain switches at the back, and try to keep the volume control at 1:30 or below.
swanny76109
382 posts
 
Interesting findings. I know ps recommends keeping the gain between 12 to 3 on the dial. I run mine at around 1:30-2 for my lomc (dyna xx2 mk2) which has .28 MV output. I use the 66 db setting and run xlr out to my pass xp10 but also have to set the pass to the higher gain setting. I know it's the weak link in my system but it works pretty well for now. What other phono pres have you used? Swanny
manitunc
795 posts
 
I dont know about the difference in gain settings, but I do know my GCPH sounds best when the volume knob is at 1:30 or less. For the $500 I paid new for the unit, I think it is exceptional. I run the GCPH straight into my amp, using the volume control on the GCPH and the result is very satisfactory. Remote control for volume, phase and mono is nice too. Also, you can really hear the differences in cartridge loading, which is external and doesnt require opening the box and flipping DIP switches. Much easier to set up the system than most.
dhl93449 OP
455 posts
 
Swanny:

I'm just getting back into vinyl (after 30+ years), so this is a "starter" phono preamp for me. I use Parasound line (JC2) and power (JC1) amps and up till now have only had CD sources (CA 840C with a Bryston BDA-1 D/A).

I considered the Parasound JC3 phono, but its 2.5x the cost of the PS Audio and I did not want to commit to that much cash at the beginning. The Parasound is also quite inflexible in cart loading choices, and gain is fixed as well.

My only beef with the GCPH so far is the cap loading (it is fixed at each resistance loading position); also it is too high for some MM carts (like the AT 150 MXL), forcing you to buy very expensive phono interconnects. Some of these cost as much as the GCPH itself. With MCs, this is much less of an issue.

My other concern was I thought this was an all class A discrete design, which it is not. Not to say that having an IC amp input circuit is a bad thing (in fact it is quite common in phono pre-amps under $1000), but their advertising implies otherwise.

I also considered the CA 650/651 products, but they need considerable modification for best sound quality. They are considerably cheaper however.

I am also considering a modification to the GCPH to remove the torrodial transformer and relocate it in an outside chassis, perhaps even building a DC supply with it and running seperate power into the GCPH. This would do a lot to reduce the noise floor even further, as I could easily see the 60/120/240 Hz power artifacts in the spectrum analysis.
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