SymAsym - "The Sequel", AAK's Rev_1.4 PCB Builders Thread

[AAK]

Hi Fred,

Don't worry about the voltage not being between 7-8V. Using the circuit that I provided in the Assembly Instructions Document find the closest match. Your jfets with Idss ranging from 6ma to 10ma should work fine.

Regards,

Al

[pchw]

Thanks you, Al !!

[coolnose]

Hi Everybody,

I took advantage of Al's offer on matched transistores as probably others in the GB.

Now I find myself with 8 2N5551 - 2 matched pairs and 4 bulk in total , but I am unsure on where to use the matched pairs.

Should I use them for Q3,Q4 or rather for Q18,Q19 ?

Maybe someone with more insight concerning the schematic could help me out here ?

Thanks in advance,

Max

[coolnose]

Matching problem solved..

Thanks!

[lpm76]

Hi Al!

Just wanted to let you know, that both channels are fired up and playing the sweet sound of Tina Dico's "In The Red"

I followed your excellent assembly instructions to the letter, and everything went well. I even hooked up a light bulb limiter as Andrew T suggested in another thread.

Power supply was fine, and measured 6mv of DC on left channel, and 15mv on the right on. I might look into that later.

Biasing was a little bit scary. Even though you stated that it would take several turns to get the bias going I got a bit spooked after the 6 or 7th turn as nothing happened. I started to wonder if I was measuring the right pins, was my meter broken, was I cranking bias like a madman into the poor OT's? :-D

Double checked everything and eventually the bias came up. I had about 1,5mv of drift on both channels.

I really like all the small LEDs It makes the circuit board seem very much alive :-)

A big thank you Al for organizing all this, and thanks to Mike Bittner, for designing the amp.

I have yet to hear it on anything than my bench speakers. I am very much looking forward to that.

[Ed LaFontaine]

Biasing was a little bit scary. Even though you stated that it would take several turns to get the bias going I got a bit spooked after the 6 or 7th turn as nothing happened. I started to wonder if I was measuring the right pins, was my meter broken, was I cranking bias like a madman into the poor OT's? :-D

I had the same experience on the previous version, including my own failure anxiety. It plays today.

[AAK]

Well, I don't want anybody having heart attack because of me. I'll test for a lower value for R34 so that in a future GB the amp comes up with a slight bit of bias, 0.1ma should do.

That's great Lars that your amps are working. Good job!

Al

[AndrewT]

Hi,
setting up a minimum of 0.1mA of bias will make this set point very susceptible to small variations in Vbe of the transistors in the multiplier and output stage.

I think a better way is to describe how to measure the multiplier output voltage. This varies continuously as the Vr is adjusted and gives a good indicator of how the setting is progressing even when the Ib is still virtually zero milliAmperes.

[AAK]

Actually I meant selecting a value for R34 so that the voltage across the OS emitters is approx. 0.1V with R36 (Vr) initially set to its highest setting. That way if it's not 0.1V a single turn or two of R36 will begin to adjust the output current without the anxiety of waiting seven or more turns.

[coolnose]

Hi Everybody,

just to let you know that we managed to finish our 4 boards last night.

They powered up smoothly and measure as follows with the 100 Ohm-resistor on the same PS-board:

V+ V- Offset

PCB1 2,99 2,52 5,5mV
PCB2 3,00 2,52 4,8mV
PCB3 2,99 2,52 5,2mV
PCB4 2,98 2,51 7,2mV

I am not sure if the imbalance on the 2 PS-rails is "normal", could Al confirm maybe ?

Furthermore, should I be worried of the one amp having an higher offset than the others ?

We did not go as far as setting the bias as the heatsinks are not drilled yet.

Cheers,

Max

P.S.: Al, referring to R44 & 45 did you have the time to do some testing ? I assume I can go ahead with the bias setting ?