Aleph P1.7 help needed

[SteveA]

I assembled the boards (from Kristijan K) some time ago. Power supply was (is) providing the correct 60V. I couldn't get the preamp to play music. After a couple of e-mail discussions with Kristijan, I sent the boards back to him (can you believe it). He said all boards were functioning properly.

I am using DACT CT2 attenuators and a Grayhill selector switch (I have successfully built the BOSOZ preamp from AudioXpress boards, DACT attenuators and selector switch). My problem has been, as I said, no music out.

When I hook up a source (CD) to the input, I can follow the signal from the input to the output of the selector switch (and to the input of the attenuators). With the attenuator moving from "lowest" to "highest" volume setting, I see a corresponding increase in the signal (ie, a variable voltage at the output of the attenuator.

So it is no surprise that this signal appears at the input on the main board. The surprise (to me anyway) is that the signal doesn't seem to be able to find it's way to the output (regardless of attenuator position).

What could/should I do to get this preamp up and running?

Thanks,
SteveA

[smario]

I've built Aleph P recently and it works fine. Check again your parts. Do you have the correct voltage on the electrolytes?
If I were you, I would disconnect everything (input selector, dact …) except preamp. Connect your CD directly to your +input, and ground your – input with wire. Connect 10k potentiometer to + output in the way that slider is your output pin, first end of the pot is connected to the + output of preamp and second end is connected to the preamp ground. If you turn the pot you must get signal on the slider. If not, something isn’t connected properly or some part is incorrectly placed or malfunctioned.

[BobEllis]

If Kristjan says the boards are working, my first thought is that you may be inadvertently shorting the outputs. (I have that Tshirt) Have you tried looking for a signal with nothing but your meter/scope connected to the board outputs?

If that doesn't help, what are the DC voltages at all pins of the gain devices (Q16 and Q19 on Pass schematic)?

Does the signal reach the gates of Q16 and Q19? (could you have miswired SW1? I don't know if Kristjan provides for this)

[SteveA]

Originally posted by BobEllis
If Kristjan says the boards are working, my first thought is that you may be inadvertently shorting the outputs. (I have that Tshirt) Have you tried looking for a signal with nothing but your meter/scope connected to the board outputs?

If that doesn't help, what are the DC voltages at all pins of the gain devices (Q16 and Q19 on Pass schematic)?

Does the signal reach the gates of Q16 and Q19? (could you have miswired SW1? I don't know if Kristjan provides for this)

smario, Bob-
Thanks for responding. I have a full wardrobe full of Tshirts. In answer to your first question, I have not tried this. I do have a signal generator, so I could send a signal that way and look at the output (what voltage signal should I use?)

I will check the voltages at the gates of Q16 and 19 and post them this evening (CDT).

Kristijan's board has SW1 but his construction manual makes no mention of it (in terms of anything on a BOM list). I assumed that meant I didn't need to be concerned about it. My knowledge of electronics is pretty limited so something that may be obvious to those "skilled in the art" is not obvious to me. That begs the question, should SW1 be used on this board and if so, what part should go there?

Thanks again,
SteveA

[BobEllis]

Well, if you haven't installed SW1, there is no need to worry about miswiring there. I believe it adjusts input attenuation, but you should be just fine without it.

Use about 1 V signal just to make thinks easy. If you are using a multimeter to trace the signal, set your signal generator somewhere near 60 Hz, since the meter's frequency response isn't likely to extend too high. If you are using a scope you can go higher. Try the "universal audio test frequency", 1 KHz.

Don't forget to ground the (-) input if your source is single ended.

[SteveA]

Hi Bob-
I have a scope (an old vacuum tube Heathkit given to me by my father-in-law who also is responsible for me having the signal generator) so I should be able to use the "universal" 1k Hz test signal.
Thanks,
Steve

[SteveA]

Bob-
Kristijan uses a different part id wrt Pass schematic. Q16/19 on the Aleph, I think, correspond to T3/6 on Kristijan's schematic (http://web.vip.hr/pcb-design.vip/aleph-17.html).

At any rate here are my voltages for:
T2(L) 15.9, 32.0, 12.0
T3(L) 4.7, 11.9, 0.6
T5(L) 14.2, 28.5, 10.3
T6(L) 4.6, 11.0, 0.6

T2(R) 14.4, 28.9, 10.4
T3(R) 4.7, 10.4, 0.6
T5(R) 14.3, 28.8, 10.3
T6(R) 4.6, 10,3, 0.6

These are G,D,S voltages in each case. The measurements bounced around somewhat for T2 and T5 (RandL) but T3 and T6 were pretty steady.

I'll chase the signal thru the preamp after the ballgame.

SteveA

[BobEllis]

It looks like a little mismatching, but it should work. T2 and T5 are the gain devices. The mismatch results in the DC offset (voltage difference between the drains of the gain devices) but since the circuit is capacitively coupled it should have little impact.

Good luck tracing the signal through the circuit.

[SteveA]

Hi Bob-
Forgot to mention, I plan on running single ended and the - inputs are grounded.

Steve

[BobEllis]

OK - You should be able to follow the signal from + input to thegate and then source of T2 through the 100R resistor - 2K pot - 100R to the source of T5 to the drain of T5. (as well as gate to T2 drain, but inverted)