Poor Man's Tube Gain Make Up Stage

[ruffrecords]

Here's a pic of the PCB assembled ready for testing.

Cheers

Ian

 

[ruffrecords]

I have now tested the board on the bench and I can confirm the mu follower works as advertised.

Cheers

Ian

 

[MatthisD]

Recieved my package this morning, much appreciated!

 

[baadc0de]

Got it here today too Nice job!

Also, would this do well instead of the both 2.2uF parts?
http://www.banzaimusic.com/Solen-Fast-4-7uF-630V.html

Cheers,
B.

 

[ruffrecords]

Got it here today too Nice job!

Also, would this do well instead of the both 2.2uF parts?
http://www.banzaimusic.com/Solen-Fast-4-7uF-630V.html

Cheers,
B.

Yes, that should be fine but you will probably have to mount it vertically.

Cheers

Ian

 

[dirtyhanfri]

Got my boards today, great package

This is a future (3-6 months if everything is ok) project, so I`ll keep looking around for a while before starting with it.

 

[kante1603]

Boards received here in germany some minutes ago!

Thank you very much Ian,

best regards,

Udo

 

[culteousness1]

Boards received here in germany some minutes ago!

Thank you very much Ian,

same here.

Best,
Carsten

 

[Albrebreton]

Boards arrived today. Looking great.
Thanks for all your work Ian, it's much appreciated.

Alberto

 

[ruffrecords]

You may recall I said this mu follower circuit would not drive a 600 ohm load but would work happily into the usual 10K bridging load, well...

If you:

1. Replace all the 1K resistors with 300 ohms
2. Short out the 220K and 10K resistors
3. Omit the 100nF capacitor
4. Replace the 12BH7 with an ECC99

you get...

An SRPP version with a 16mA standing current that will put over +17dBu into a 600 ohm load with a gain a fraction over 20dB so just enough to make up for the poor man's EQP1A insertion loss. Being an SRPP its distortion is not as good as a mu follower but good enough I think. I measured at 6V rms into 600 ohms (+17.78dBu):

2H 0.75%
3H 0.45%
4H 0.022%
5H 0.005%

At normal operating levels of +4dBu these will drop proportionally to around 0.2% 2H and 0.1% 3H. Just enough for a nice warm tubey sound. This same circuit will drive a whopping +32dBu into 10K. Obviously you will need to make changes to your power supply to provide the extra current (both HT and heaters) and the output coupling capacitor will need to be increased somewhat for a 600 ohm load

Cheers

Ian

 

[baadc0de]

Is this SRPP a schematic we discussed here? I'm sorry if we did and I didn't find it..

 

[ruffrecords]

Is this SRPP a schematic we discussed here? I'm sorry if we did and I didn't find it..

Yes and no. It is actually the SRPP stage of the 'improved headphones amp' thread so we have not discussed in this thread but it has been mentioned in another thread I have attached the phones amp schematic. I am just talking about the ECC99 stage on its own.

Cheers

Ian

 

[MatthisD]

Would it be viable to ad a mic input in front of the gain makeup? (..albeit without phantom and suited for loud sources)

Assuming a Poor Man's pultec/gain makeup combination and adding a 2nd XLR input wired to a 1:5 or similar mic input transformer with the secondaries wired to the third position of the EQ bypass switch. So you'd have 'EQ in', 'EQ bypass' as in Ian's schematic and a 'Mic Pre' position. Could you simply use a gain pot for 'microphone level' at the makeup gain input and a loading resistor back to input transformer secondaries, would this allow for full use of the Poor Man's 26db and the stepup gain from the input transformer?
cheers
Matt

 

[ruffrecords]

Would it be viable to ad a mic input in front of the gain makeup? (..albeit without phantom and suited for loud sources)

Assuming a Poor Man's pultec/gain makeup combination and adding a 2nd XLR input wired to a 1:5 or similar mic input transformer with the secondaries wired to the third position of the EQ bypass switch. So you'd have 'EQ in', 'EQ bypass' as in Ian's schematic and a 'Mic Pre' position. Could you simply use a gain pot for 'microphone level' at the makeup gain input and a loading resistor back to input transformer secondaries, would this allow for full use of the Poor Man's 26db and the stepup gain from the input transformer?
cheers
Matt

I assume you intend the mic pre to work only without the EQ in which case I would say that would be fine. In fact I use that very same mu follower stage in my mic preamps. It will give you about 26dB of gain and a 1:5 transformer will provide an additional 14dB for a 40dB total which should be fine as you say for loud sources. I suppose without the EQ we could call it a poor man's tube mic pre 

Cheers

Ian

 

[MatthisD]

Yes exactly, an extra pair of poorman's preamps for just the cost of 2x transformers and some bits and bobs. You could always use one channel for the Mic preamp and the other for EQ (out and back in). Should the gain pot be 470k Log rather than linear?
thanks alot
Matt

 

[ruffrecords]

Yes exactly, an extra pair of poorman's preamps for just the cost of 2x transformers and some bits and bobs. You could always use one channel for the Mic preamp and the other for EQ (out and back in). Should the gain pot be 470k Log rather than linear?
thanks alot
Matt

For the EQ it should be linear because you should only need to alter the gain over a limited range. That way you will have about 6dB attenuation at the half way point. As the EQ has an insertion loss of about 20dB and the amp has about 26dB of gain then the pot will usually end up somewhere near the middle of is travel. A log pot would give about 20dB attenuation at the mid point which is not what we want.

Cheers

Ian

 

[MatthisD]

For the EQ it should be linear because you should only need to alter the gain over a limited range. That way you will have about 6dB attenuation at the half way point. As the EQ has an insertion loss of about 20dB and the amp has about 26dB of gain then the pot will usually end up somewhere near the middle of is travel. A log pot would give about 20dB attenuation at the mid point which is not what we want.

Cheers

Ian

Cheers

ian

I had noted that in the first post about the Linear pot for the EQ, I was referring to the 'gain pot' that could be used for Mic level. But as you said a Log provides 20dB attenuation at midway point, sounds about right.
Much appreciated
matt

 

[ruffrecords]

I had noted that in the first post about the Linear pot for the EQ, I was referring to the 'gain pot' that could be used for Mic level. But as you said a Log provides 20dB attenuation at midway point, sounds about right.
Much appreciated
matt

I think it was because you said 470K that I got confused. If you are putting in a separate pot for the mic gain then it would be best as a log pot. However, 470K is probably not the best value for it. For optimum response and to present the right input impedance to the microphone, the transformer would probably be better loaded with something in the region of 150K or even less depending on the transformer.

Cheers

Ian

 

[danijel]

Got the boards today.

Thank you!

 

[MatthisD]

Hi again, I'd just like to ask if the PSU can handle anymore than 2x 6cg7 with the right transformer?

Also, will a 10k/600ohm output transformer enable this to drive 600ohm loads without the adjustments mentioned a few posts earlier?

one more...with the EQ input impedance being 10k could I take the signal from the output of one gain stage straight to the EQ input?

I suppose what I'm aiming at is a one-bottle type mic preamp with PM Pultec/gain makeup.
cheers
Matt