first attempt at the NP I/V stage

Bill Fuss · 29th October 2007, 12:10 AM

Well, I made a couple boards and installed them in a Rotel 971, fired it up and checked everything out. Input offset zeroed out and all voltages where they are supposed to be. It sounds great, very transparent and deep, but the output level isn't anywhere near where I thought it would be.
This is my first attempt at any circuitry around a dac chip and I either missed something basic or I need another gain stage. On a -10db test disc I have around 25mv output.I have the Iout and BP0 pins going straight to the board inputs and picked up the ground near the PCM63s.
Somebody straighten me out, you can even yell if you want.

Bill

hifimaker · 29th October 2007, 03:45 PM

A few questions:

What DAC Chip are you using?
What is the DAC's current output?
Let us know the MOSFET's voltage supply.

-David

Bill Fuss · 29th October 2007, 08:40 PM

I am using all the rotel circuitry up to and including the PCM63s. I have a 25VA transformer and regulated PS set at +/-28v for the I/V board. I have lifted the jumpers that connected the Iout and BP0 pins on the PCM63s to the stock output stage, tied them together and fed that point to the input on the new board.
The PCM63s are supposed to put out +/-2ma which is the design parameter of Mr. Pass' circuit. I believe I have it hooked up exactly as intended but checking the signal level at the board input with a test disc playing the -10db track I am reading about 4mv. Is there a mute function that I have inadvertantly enabled when I Isolated the outputs or does that sound like a normal signal.
I also experimented with a 100ohm resister load and got the same signal level, about 4mv. I realise they are current outputs but shouldn't the load voltage be higher? The thing is when I put it back to stock configuration it plays fine, and I've done that 3 times.
As I said, the new board sounds really great but the level is so low it is not practical to operate with the rest of my system,so in other words the operation was a sucess, but the patient died.
Anybody got any theories or any more questions?
Bill

hifimaker · 30th October 2007, 04:16 PM

Since the new I/V is working, just not loud enough, I'd look at the resistors you have for biasing the buffer FET's.

When I sim'ed the D1 I/V, the diffeerence between 28 and 30 vdc supply is about 1 volt on the output.

Try dropping the values of R28 and R34 from 1k5 to 1k. This should raise the output voltage about 3 volts with a +-2ma source and a 28vdc supply.

-David

Bill Fuss · 30th October 2007, 09:11 PM

Thanks David, I'll try it tonite and report the results. Much appreciated. Bill

ctong · 3rd November 2007, 08:34 PM

Are you using the original DAC on the Rotel? What is the output current and the center current?

Bill Fuss · 4th November 2007, 01:33 AM

I haven't had much time to mess with it the last couple of days but the buffer section is just a source follower so there isn't any gain in that section, unless I'm mistaken.
The Dacs are PCM63s- output is 0 to -4 plus +2, equals +- 2ma.
Might not be turning on because of too low a supply, going to try larger trans, wont fit in case but if its successful I'll buy a new one that will.
Any other questions or input?
Thanks all, Bill

29th October 2007, 12:10 AM	#1
Bill Fuss RIP Join Date: Oct 2005 Location: Central Florida	first attempt at the NP I/V stage Well, I made a couple boards and installed them in a Rotel 971, fired it up and checked everything out. Input offset zeroed out and all voltages where they are supposed to be. It sounds great, very transparent and deep, but the output level isn't anywhere near where I thought it would be. This is my first attempt at any circuitry around a dac chip and I either missed something basic or I need another gain stage. On a -10db test disc I have around 25mv output.I have the Iout and BP0 pins going straight to the board inputs and picked up the ground near the PCM63s. Somebody straighten me out, you can even yell if you want. Bill

3rd November 2007, 08:34 PM	#6
ctong diyAudio Member Join Date: Jun 2005 Location: Clemson, South Carolina	Are you using the original DAC on the Rotel? What is the output current and the center current? __________________ CT

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29th October 2007, 03:45 PM	#2
hifimaker diyAudio Member Join Date: Oct 2003 Location: San Jose	A few questions: What DAC Chip are you using? What is the DAC's current output? Let us know the MOSFET's voltage supply. -David

29th October 2007, 08:40 PM	#3
Bill Fuss RIP Join Date: Oct 2005 Location: Central Florida	I am using all the rotel circuitry up to and including the PCM63s. I have a 25VA transformer and regulated PS set at +/-28v for the I/V board. I have lifted the jumpers that connected the Iout and BP0 pins on the PCM63s to the stock output stage, tied them together and fed that point to the input on the new board. The PCM63s are supposed to put out +/-2ma which is the design parameter of Mr. Pass' circuit. I believe I have it hooked up exactly as intended but checking the signal level at the board input with a test disc playing the -10db track I am reading about 4mv. Is there a mute function that I have inadvertantly enabled when I Isolated the outputs or does that sound like a normal signal. I also experimented with a 100ohm resister load and got the same signal level, about 4mv. I realise they are current outputs but shouldn't the load voltage be higher? The thing is when I put it back to stock configuration it plays fine, and I've done that 3 times. As I said, the new board sounds really great but the level is so low it is not practical to operate with the rest of my system,so in other words the operation was a sucess, but the patient died. Anybody got any theories or any more questions? Bill

30th October 2007, 04:16 PM	#4
hifimaker diyAudio Member Join Date: Oct 2003 Location: San Jose	Since the new I/V is working, just not loud enough, I'd look at the resistors you have for biasing the buffer FET's. When I sim'ed the D1 I/V, the diffeerence between 28 and 30 vdc supply is about 1 volt on the output. Try dropping the values of R28 and R34 from 1k5 to 1k. This should raise the output voltage about 3 volts with a +-2ma source and a 28vdc supply. -David

30th October 2007, 09:11 PM	#5
Bill Fuss RIP Join Date: Oct 2005 Location: Central Florida	Thanks David, I'll try it tonite and report the results. Much appreciated. Bill

4th November 2007, 01:33 AM	#7
Bill Fuss RIP Join Date: Oct 2005 Location: Central Florida	I haven't had much time to mess with it the last couple of days but the buffer section is just a source follower so there isn't any gain in that section, unless I'm mistaken. The Dacs are PCM63s- output is 0 to -4 plus +2, equals +- 2ma. Might not be turning on because of too low a supply, going to try larger trans, wont fit in case but if its successful I'll buy a new one that will. Any other questions or input? Thanks all, Bill

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