Helios Mic Preamp
- Thread starter Gearsix
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ruffrecords
Well-known member
The standard little three transistor circuit you see on the web is the mic preamp. It provides up to 40dB gain and with the 1:10 mic transformer gives a total gain of up to 60dB. The nominal output level is about -10dBu. There is usually about 10dB of extra gain down stream giving a total available gain of 70dB. Downstream is usually the Helios passive EQ (like my Helios 69 design) and another of the three transistor amps acting as gain make up. Helios mic pre modules were not designed to drive a direct out but to drive buses. Line outs came after the mix amps and there is a different amp ( a proper line amp) intended to drive high levels into 600 hm loads via a transformer.
Cheers
Ian
Cheers
Ian
ruffrecords
Well-known member
warpie said:
I don't think so because it is straining at the leash with 40dB gain as it is. That is about all you can sensibly get with a simple three transistor circuit. Neve had to use two such amps to get 80dB of gain in their mic pres and even they used a 1:2 mic input transformer.
Cheers
Ian
ruffrecords
Well-known member
Gearsix said:
The simplest way is to follow the three transistor mic preamp stage with the Helios line out stage and then you can probably use a 1:1 or maybe even a 1:2 output transformer. The Helios line amp is the 22113 and there is a thread about it here:
http://groupdiy.com/index.php?topic=59428.msg754727#msg754727
The schematic is available on line.
Cheers
Ian
Ian, I have found a schematic where the output circuit is built around only one transistor
Is it possible that the 22113 preamp is intended to compensate the loss of the equalizer?
Considering that I want to realize only the preamp what do you think to put in the output this simpler circuit?
Is it possible that the 22113 preamp is intended to compensate the loss of the equalizer?
Considering that I want to realize only the preamp what do you think to put in the output this simpler circuit?
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ruffrecords
Well-known member
Gearsix said:
All that does is provide a high input impedance buffer for the HPF that precedes it
I don't think it was intended for that but it could be used that way. Its primary purpose is to drive a transformer to provide a balanced output from a mixer. It has gain so it could be used as a gain make up amp.
You could put a slightly different 3 or 4 transistor circuit with little gain but a push pull output stage to provide the required output drive.
Cheers
Ian
In the original version the output current is limited by the 1.2k collector resistor. In the second version, the current is limited by the pseudo constant-current source constituted by bottom transistor (MPS651). Although it's admittably better, I don't think it's really adequate for driving a transformer. IMO you need a real push-pull or a much higher quiescent current. Indeed that depends on the transformer, but I'm just ASSuming the use of a standard OT rated at 600 ohms.Gearsix said:
ruffrecords
Well-known member
I agree with Abbey. It is just an emitter follower tacked on the end of the original circuit and equipped with a sort of constant current load. Adding a diode and a resistor would turn into a better constant current load but even then it is not really up to the job of driving a transformer. A more conventional approach would be to drive a complementary class B output pair from the third transistor. I'll see if I can find a suitable circuit.
Cheers
Ian
Cheers
Ian
ruffrecords
Well-known member
Attached is a scan of the last page of an article by David Rees dated 1971. I think he worked for Alice or Audix but I am not sure. The circuit at the bottom of the page has a lot of similarities to the Helios one but it has a push pull output stage that will drive +20dBu into 150 ohms. With a suitable 1:2 transformer this should be able to drive +26dBu into 600 ohms.
Cheers
Ian
Cheers
Ian
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Ian, I have added to the schematic the polarity of the capacitors but I am not sure about the diodes
Do you think that a couple of 1N4148 are fine?
Do you think it is better to put a pot between the output of the Helios mic stage and the input of the line out or to use a rotary switch in the RFB position?
Do you think that a couple of 1N4148 are fine?
Do you think it is better to put a pot between the output of the Helios mic stage and the input of the line out or to use a rotary switch in the RFB position?
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ruffrecords
Well-known member
If you need the full 70dB gain then the simplest method is to put a pot between the preamp and line out stages. A 10K log pot will do. If you use a 150:600 output transformer then you get 6dB of gain from the transformer so you only need another 4dB from the line out amp. So, if 66dB gain was enough for you then you could eliminate the preamp stage altogether. The line out stage can provide 40dB of gain, the input transformer provides 20dB and the output transformer provides 6dB. This would make a very simple mic pre.
Cheers
Ian
Cheers
Ian
Who needs 70dB gain today?ruffrecords said:
I'm only half joking. I have much more mileage from preamps having their soft spot around 35-40 dB than any other.
Now I don't do any classical music, but even doing folk/trad, I never need that much gain.
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