Altec 1214A - Balanced Outputs Mod

[smilan]

Hi, I have an old Altec 1214A mixer amplifier.
I disconnected the power amp and I'm only using the preamp and reverb circuits.
For some applications I love the sound of those preamps and EQ so I would like to be able to use all the preamps with more gain and with an external output.
With the preamp and the summing mixer I'm getting a gain of about 40db at the Monitor out.
I would like to build another stage of amplification to get closer to a standard 60db preamp and add a transformer coupled output.
I would like to power it with +24V that I have available from the existing PSU.
I would love to hear any suggestions for the additional circuit?
Attached schematic of the original preamp card and the summing mixer.

 

[ruffrecords]

The first stage of the preamp appears to have a gain of about 6dB. The EQ section looks to have about 12dB gain giving a total so far of 18dB. The first stage input impedance is very high and the ALtec literature suggests for mic inputs they usually fit a 600 ohm mic input transformer. It would not be unuauals fot such a transformer to have a 1:10 ratio which would provide an additional 20dB of gain.

The puzzle is the output/mixer stage. This appears to be configured as some kind of virtual earth mixer. Its input would therefore normally be fed via a series resistor but instead you have connected the preamp output directly to it. This may well produce the additional gain you see but it will heavily load the output of the EQ stage which will limit dynamic range and probably increase distortion.

it would help if you could post the full schematic of the original mixer.

Cheers

Ian

 

[smilan]

Thank you Ian,
The transformer gives about 12db of gain (I tested it with a Boonton 1120 with the oscillator's 600 Ohms balanced output connected to the transformer's primary and J1 connected back to the Boonton input).
Attached the full service manual of the 1241A mixer.
If I not mistaken, there's a 10K summing resistor between pin 4 (output) of the preamp card to the positive side of C10 at the summing mixer.

 

[ruffrecords]

The service manual certainly clears things up. Yes, you do need the 10K feed resistor at each input of the mix amp. Its gain with then be unity.

The AUX input is interesting because it modifies the standard input amp so it can have two gain settings. It might be worth modifying the mic channels to do the same to give you some more gain.

Cheers

Ian

 

[smilan]

Thank you Ian,
With this configuration (with the 10K resistor) I'm getting 41.3db of gain.
I would like to add extra 20db of gain and drive a balanced output transformer.
How can I make it?