PRR Vari-mu bias supply voltage criticality?

Hi,

Finally getting time to build the PRR varimu... and I have a question.

Checked voltages on the power supply unloaded. Everything's cool except the bias supply for the TL074. I got 1.38 VDC, whereas it should be at least 1.5 and I think others in an old thread have mentioned 1.55 or 1.6 (Svart and PRR, respectively).

I understand that changing the LED could result in a different (hopefully higher/correct) voltage, but instead of sorting through a bunch of LEDs (of which I have none right now, anyway), what would the consequences be if I left in that particular LED? In other words, how critical is the bias supply voltage in this circuit?

Thanks,
Chris

Hmph.

No one responded to me so I went down to the local Rat Shack and got a bubble pack 1.7V LED to put in.

In the circuit (unloaded), I get 1.63VDC. I suspect that value is better than 1.38VDC. ?

Anyway, stayed up until 2am last night finishing up what I could. I still have to wire up the power switch, pop in the opamps and tubes, connect to my console, fire her up and tweak.

Will let y'all know how it turns out.

OK, powered it up, seems like it's working. It was late here, I couldn't put program through it and try it out extensively, but I can see the gain reduction on my console's meters when I tap the mic (the meters on the PRR are not working, will take a look tomorrow).

Here's a pic of the front panel:

Looks great!
:grin:

it look great!

If I`m right the 1,5V bias control the knee point of the limiter, I also can`t find a 1,5V led but it work ok for me, anyway you have a threshold control .
the meter is a uA meter? what audio transformers did you use?

Rafael

:thumb: Looks really great ! Do you have any inside pics as well ?
I'm thinking of building one to use on the master bus.

Also interrested in what you think of the sound, and what trafos you used.

:guinness: :guinness:

[quote author="12afael"]
the meter is a uA meter? what audio transformers did you use?
Rafael[/quote]

The meters are 1 mA, as called for by PRR.
Audio transformers are the Radio Shack units: "Using the car-sound transformers, the sound should be good..."

[quote author="MrZpliff"]Do you have any inside pics as well ?
[/quote]
Not right now, it's a mess inside. After I shorten the wires and tidy 'er up a little bit...
[quote author="MrZpliff"]
Also interrested in what you think of the sound, and what trafos you used.
[/quote]
I've yet to put program material through it (I just tested with one hand tapping on a mic to see if it worked; it was really late last night and everyone was sleeping).

OK, got the meters working. I had the terminals wired backwards.

I tried adjusting R32 for minimum "thump" (DC bias shift) as suggested by PRR but could not hear any differences at all when I adjusted the pot...

As kent and PRR mention, high gain reduction sounds mangled.

Overall, I'd say the sound is fairly transparent. I am liking it.