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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
michelzay
106 posts
 
Hi there,

I purchased a new PS GCPH a while ago for MC cartridge usage and just used it in my system, which consist of a Mcintosh Mx-135, Denon Dp-A100 TT, Mcintosh MC402 and B&W 804d.

1. When I use ps gcph+amplifier, without another pre-amp, the sound is fine, but the MX135 with its MM is much better!
2. In connection mode “without PS gcph”, Denon DP-A100 (TT)->MX-135->MC402, the whole system sounds wonderful with MM cartridge (AT440ML). However when I put PS GCPH into the chain such as, Denon DP-A100 -> PS GCPH ->MX-135->MC402, the sound quality degrades enormously and gets lean where I loose some details such as imaging, staging, edges on Bass and midrange frqs. I still didn’t try MC cartridges with PS gcph+preamp but I guess I will have the same effect.

As a matter of practicality of using one main cable set with my amp and not switch it between Ps gcpj and pre-amp, I was wondering why using PS gcph+preamp degrades the sound enormously and what should I do to gain the sound quality that I would expect from ps gcph?

As for input level informations on MX135, here is the data:

Input Impedance
Phono: 47k Ohms, 65pf
High Level: 22k Ohms Unbalanced
50k Ohms Balanced

Sensitivity for inout
Phono: 5mV
High Level: 400mV Unbalanced
1V Balanced

Maximum Input Signal
Phono: 50mV
High Level: 5V Unbalanced
10V Balanced

I would appreciate your feedback. I tried PS audio support desk and so far there is no result!?!
dhl93449 OP
438 posts
 
I suspect the input capacitance of the GCPH plus your phono cables is too high for that cartridge, but that is just a guess. Its not published, but PS told me the input capacitance for the MM/47K input is 100 pF. I have an AT 150 MLX that requires a max of 150-200 pF, and is reported to sound bright and edgy with higher capacitance. 150 pF is reported to be the optimum, but with the GCPH 47K input of 100 pF and an arm wiring capacitance of 30 pF, it leaves only 20 pF for the cables. Almost impossible to obtain (unless you want to pay thousands for the interconnects).

The Mac may sound better because it has a much lower input capacitance at 65 pF.

In my case (see my post under analog on the AT 150) I solved my problem but using a 5:1 attenuator ahead of the MM input on the GCPH. This isolates the 100 pF input cap from the cartridge, and now I can use 100-120 pF cables without exceeding the 150 pF loading on the cartridge.
I use a gain setting on the GCPH of 60 dB to compensate for the attenuator.

The attenuator is easy to build. Its located in an inline RCA adapter that plugs into the GCPH inputs.
dhl93449 OP
438 posts
 
Michelzay:

Forgot to ask, what inputs are you plugging the GCPH into when you use TT>GCPH>MX135? A high level Aux input, right? The same one you use when you use a MC cartridge with the GCPH?

You of course cannot connect the GCPH into the phono stage of your MAC pre-amp. That would be disasterous (you are double RIAA EQing!)

If you are going into an AUX input on the MAC, perhaps therein lies the problem. Maybe you are overloading the inputs with the output from the GCPH. Does the sonic degradation stay the same if you turn the level control of the GCPH way down and use the MAC for volume adjustment?
michelzay
106 posts
 
Since a few days ago I left GCPH on and connected to the system, for break-in purposes. I tried again this morning and I was amazed how sound quality was improved. Perhaps after more break-in process the sound quality will improve. I read somewhere it would takes about 300 hours!. As for gain/sound adjustment, I use 48db knob and switched to 66 db where I put volume on 1:30-2 at GCPH, as it is recommended in the web sites. the result is much better in this case.

perhaps with more break-in process, the sounds will improve further. As a comment the PS audio has been very supportive ... thanks ...
dhl93449 OP
438 posts
 
66 dB? Why do you need so much gain for a MM cart? At 66 dB and 1:30 to 2 you are getting about 60 dB total gain which is x1000. For a nominal 4 mV (at 1K Hz) MM cart output, that is 4 volts output!

That's a lot for a line amp input.
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