op amp following 12ax7 tube

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Andybot

Well-known member
Joined
Sep 7, 2004
Messages
105
Hi all,
i need some advice and help.
I would like to amplify the outputs of a 12ax7 tube with an op amp so i can convert my altec 1567a into a four in four out pre.

I have done some research but am still a little lost.

I want to use the quitest cleanest op i can but im a little lost on how to choose one, or build one, and the best way to hook it. So im basically in need of some advice.

Any help is really appreciatted.

Thx
 
Thx NYD,
i was thinking of building a seperate box to sit above the altec with the op amps and the outs.
If its possible I would take power from the altec somewhere or build a seperate supply.

Having never tried this im sorta just blowing smoke.

Why an FET input op?

Thx
Andy
 
[quote author="Andybot"]Why an FET input op?[/quote]

I'm guessing to match the high impedance of the 12AX7s output.
 
Hi and thx.
I was thinking of taking the signal right after gain pot for each channel , before or after the 330 k resistor, or somewhere in the middle of the resistor.

My thought was if i have enough consistent gain from the op amp then i could use the original gain switch.

Is that a good place to take my signal from?

I used a .022 cap to take a signal from just after this resistor and have a signal that sound like i want it to but needs a ton of gain. It was peaking out at about -20db. I just took the signal out as if i was signal tracing, so i didnt unhook anything.

Thx for all the replies.

Andy
 
The impedance at the point you refer to in the unit can't be greater than about 120K Ohms (220K plate load in parallel with the 250K gain pot). Using a bipolar opamp like a 5532 (330K input impedance) would load the circuit too much and wouldn't allow enough gain since the input resistance would be small. I think that is why an FET input op-amp was suggested. You could use a bipolar DOA like a 990C though. That has an input impedance of greater than 10 MOhm.

You would want to take the signal before the 330K Ohm resistor.
 
Thx for the imformation.
I will do a few tests this weekend.

While im here im gonna change the caps and resistors in the signal path.
Any suggestions on caps and resistors that are considered quiet and usefull for this kind of application.

Thx Again, Andy
 
Hi all.
I did a test taking the signal from before the 330K resistor and padding the ground with a 150K resistor.

The peak level was around -15dB with the gain on full.

This is the point where im lost.

Is it a matter of selecting the correct op amp, powering it and setting it to a specific gain allowing me more freedom with the gain control.

Or am i off base.

Thx
 
Sorry if I ask stupid question, but I am not sure if I understud your meassurings.
You meassured 15dB on what scale? dBu dbV?
And what level or source did you connect at the Input while your testing?

Also try to find a good beginners reading about Op-Amps. I did a quick search in the Meta-Meta (seen above) , but I did not find an explaining as simple and straitforward as in most of the "Elektronic Basic´s" books I know.
I would reccomend to find a good book. It will be your friend for a long time. I am not native english (an one could easily see on my spelling), so I can´t recommend someting. Sorry

Do you know how much more gain max., than your acctual level is providing, you want?

Someone said FET-Op amp would be good as it has high imput restistance and therefor is not a heavy load to your circuit. If it seems to be your first Op-Amp projekt I would not care about too many options and just use any known quality standard component.

I know I could not give you all your desired answers, but maybe we get one step further.
 
If I understand correctly, you're trying to take the input stages of a mixer and break them out as individual preamp outputs, using an opamp?

One problem you may run into (aside from the mundane issue of needing +/- supplies to run the opamps) is the behavior of opamps when driven by high-impedance sources like the plate of a 12AX7. Many opamps, if used in a non-inverting unity-gain configuration, will generate high distortion levels under these circumstances, much higher than they would when driven by lower impedances. The culprit is nonlinear input capacitance; it's particularly acute in FET-input opamps, but it's also present in bipolars like the NE5534. According to my tests, the effect is not present in the LM310 buffer amplifier; perhaps that's going to be your answer.

Peace,
Paul
 
Hi DerEber,
thx for the helpfull info.
I feed the out into a blackface adat machine and with full gain on the altec and my friend singing something that we know would be a peak in a normal recording session. I was using a mic at the input, an equitek E200, and although not a signal genertaor my friend knows what shes doing when it comes to singing.

So my reading is -15dB from full scale, where it starts to distort.

I have a basic understanding of op amps but the projests i have built are
set out for me and designed by other people but im comfortable with building
the things i need even if they are a challenge.

I will read some more though as my goto book gives a very basic overview.

Thx Again DerBer

Hi Paul, thx for your input.
As Derrber suggested I was going to go with an op amp that I know will work so thanks for the suggestion of the LM310.

I was thinking there might be a way of taking power from the altec to run
the op amps, i have built power supplies for these before but my preference would be to take it from the altec. Again im not sure if this would be a problem or not.

I know that this mod has been done by other people so i know its possible but theres no info online about it or what was used in the mod, its whay im here i guess.

Thx again both of you, i appreciatte your time .
Andy
 
If you've been feeding the 12AX7 directly to the ADAT then yeah, I'd expect a problem. 12AX7 voltage amplifiers aren't good for driving 10k loads.

I don't know what supplies the Altec has available; I'd be worried about pulling too much current if you tried to run the opamps from the plate supplies. And the filament supplies, if it uses DC filaments, are too low-voltage.

What about building a small box with the buffer amplifiers and a +/- supply, to go on top of the Altec?

Peace,
Paul
 
So the more you turn the "gain" on the pot up, the more you paralell your Blackface input resistance (10k?) next to 250k. At full clockwise you will load your tube with 9,xKOhm instead of 250kOhm wich will lead to a drastic change in loadline and this is why you expirience distortion when turning the level up. This will not hapen if you buffer with the right op-amp solution.

If you are interested in learning about tube load lines I can recommend these readings:
http://www.audioxpress.com/resource/audioclass/index.htm

If you want to know how much level you get, without loading the tube to much by your Adat, you need to find another meassuring methode. Or you start experimenting when your Op-Amp circuit is allready there. Both will likely work. If you care about how to meassure things the right way one is better. :green:

Another question:Did you notice any High-Frequency loss by taking the signal out there?
 
Assuming that you have the 4722 microphone transformer on the input set for 150 ohms, the overall gain of the transformer and first preamp stage is approximately 70db. Your microphone has a sensitivity of -55db (ref 0db = 1 volt per microbar). This is a sound pressure level of 94db, probably a bit louder than your singer could normally produce. If you are planning on running the output into a -10db input, assuming that this is -16db from the point at which your AD converter overloads, then the maximum input level you need to produce is +6db. If you are getting -15db now and you want this to produce approximately a 0db level out (to give a bit of headroom over your maximum level from the pre) it appears you need about 15db of gain in a second stage. You also need to derive some drive capability from that stage to drive the relatively low input impedance of the converter.

To build an opamp circuit with these characteristics and give about a megohm of input impedance, you would use a 1 Meg input resistor and a 5.6 Meg feedback resistor. You can use the inverting input of the opamp since the single tube gain stage will invert. The output from the two stages will be in phase with the input. Be sure to place clamping diodes on the input of the opamp stage that will clamp the input to the plus and minus opamp supplies. This will prevent the DC charge on the coupling capacitor from destroying the opamp during power up and shutdown. Use a seperate supply to provide the + and - 15V for the opamp stages. Decouple the power inputs at each opamp with 47uf electrolytics shunted with .1uf monolythic caps. I would suggest trying it out with some cheap FET opamps like LF353 or TL074. Both of these will have significantly better distorition characteristics than the transformer/tube combination preceding them.

Loading of the output of the AC coupled tube plate will make only a small change in the loadline of the tube which is a DC phenomenon related to the grid to cathode voltage and the size of the plate load resistor and plate current. Loading after the coupling capacitor will decrease the impedance that the tube sees on the output (an AC effect) which will cause the tube to try and deliver more power to the output load. The audible effect of this will be to increase the distortion level since the AC waveform will droop as the voltage increases from zero.
 
Hi,
first thx for the replies.
Second i gotta run to work.

I did place a 150K ohm resistor on the ground side of the line I took out so I wouldnt have
have such a huge mismatch.

My problem was never with distortion, just the amount of gain i have.

Thx Again.
Andy
 
This guy had basically the same idea with Bogen tube mixer. I think he covers the basic considerations that have been discussed so far.

http://www.clarkhuckaby.com/BogenMod/MxmBufIm.html
 

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