PL2000 preamp square wave ringing.

[George S.]

Couldn't sit around waiting on parts.
Grabbed my main preamp out of the system and swapped the 33 pF for 15 pF.
No joy.
Back to waiting on the 7 pF to arrive.
(I did have the Ambience pot wired correct on this one)
Sat up until midnight last night listening to Lee's favorite, Infected Mushroom, on the preamp with the 7 pF caps.
Installing them totally transforms the unit. Very worthwhile.

 

[mlucitt]

Are you getting more 7pF caps from the same guy on ebay? It is a good deal. I may order some more.

 

[George S.]

Are you getting more 7pF caps from the same guy on ebay? It is a good deal. I may order some more.

Mark L., no, I got 2 orders for a total of 80 pcs, all NOS Cornell Dubilier. A extremely good deal, but he didn't send any 7 pF. Did send 10 and 5 pF.
I only had two very old 7 pF, generic, with no manufacturer mark, that tested very good on the DER EE.
So I ordered these.
https://www.ebay.com/itm/2015469624...NB_S8D7Qg2&var=&widget_ver=artemis&media=COPY
I don't like the 500 volt leads, 300 volt are easier to work with, but the spacing is usually correct with the 500's.
Today I'll check my results with the newer digital equipment as well.
That old HP equipment is just much more fun to use.

 

[laatsch55]

Mark, I try to standardize at 150 mv input..

 

[George S.]

I've been using 400 mVpp output on the HP3325A which gives a square wave voltage of appx 380 mVpp at the cables end RCA.
That 400 mVpp output gives a sine wave output of appx 260 mVpp at the RCA, so "it depends".
I strive for unity gain, then that extra to get over 0 dB on the receiving instrument. Probaly about 2 Vrms gain if I'm understanding correctly for single ended outputs on a preamp.

 

[George S.]

Waiting on parts, so let's generate the square wave with the HP3325A and look at it with the QA403.
First up is a original spec unit with a ceramic 33 pF, then a unit with the silver mica 7pF.
Both units have the tranformer and rectifiers installed as evidenced by the 60-120 Hz noise.
Pretty easy to see why the square waves aren't square on the scope.
10 mHz square wave input.

 

[George S.]

20 mHz square wave. Yeah, I ran these measurements out to 60 kHz.

 

[George S.]

Looking at the main systems PL2000S2 that uses a offboard Power-One power supply. A additional ground wire from the preamp to the power supply enclosure is beneficial.
The power supply "floats" in it's enclosure with no direct ground. The enclosures IEC socket ground lug does have a direct connection to the enclosure.
The added ground wire will be the main ground wire for the entire system.
The 30 and 90 Hz spikes are enviromental noise.
Looking at a few things before I swap in the 7 pF caps on this unit. 15 pF are presently installed.

 

[George S.]

Decided to try shielding the power cable and utilize it for ground.
Worked well.

 

[Gepetto]

Decided to try shielding the power cable and utilize it for ground.
Worked well.

Yup, down over 120dB across the spectrum. Good job George

 

[George S.]

We're getting there Joe!

 

[George S.]

Mark L., told ya a while back I'd run some frequency response sweeps. Here they are. Big difference between a unit with the 33pF ceramic and a unit with a 15pF mica. Small difference between the 15pF and 7pF at about 25 kHz. I ran these out to 40 kHz.
And, a sweep of the benchmark, the Acurus L10.
I'm going to keep at this. I have some small opamp pcb boards on the way. Going to learn what I can about making these preamps better.

 

[Gepetto]

Mark L., told ya a while back I'd run some frequency response sweeps. Here they are. Big difference between a unit with the 33pF ceramic and a unit with a 15pF mica. Small difference between the 15pF and 7pF at about 25 kHz. I ran these out to 40 kHz.
And, a sweep of the benchmark, the Acurus L10.
I'm going to keep at this. I have some small opamp pcb boards on the way. Going to learn what I can about making these preamps better.

Hard to beat the Acurus George...and I love the way it looks

 

[mlucitt]

And, a sweep of the benchmark, the Acurus L10.

How do they get the low frequency so flat!!!

 

[George S.]

I don't know, but I'm going to find out.
Been reading Rod Elliotts' documents he's posted on his ESP site, about designing with opamps.
Granted, there are none in the Acurus.

 

[J!m]

How do they get the low frequency so flat!!!

You should ask that question of Wyn Palmer.

 

[Gepetto]

How do they get the low frequency so flat!!!

I will tell you George...let me look at the schem and get back to you

 

[Gepetto]

Increase the value of C42 and C43, go from 10uF to 100uF and you will notice a difference.

 

[Gepetto]

Change C21 and C22 from 0.22uF to 2.2uF and you will notice a difference

 

[Gepetto]

How do they get the low frequency so flat!!!

much better design work, PL really cheaped out and wanted to do everything in one quad op amp instead of adding a buffer stage earlier on to isolate the effects of the volume and balance controls.