IC power amplifier as a microphone amplifier… any experiences?

¿Has anyone used power amplifier IC as the end of a microphone preamp?. I always thought this, seeng that the schematic of the module 990 is very similar to a conventional power amplifier.

If you would like more gain you can use another step with a single OPA, even, eliminate the output capacitor with a servo, and use input and output transformers.

WAW... a cheap Millenia or API... ;D

Any experiences?

no experience, but first thoughts are to noise level, and amount of gain.

I've used a TIP120 65-Watt power transistor as a mike head-amp.

What Rochey says. You need to ponder the input noise level. Some "power amps" are quite good, at around 10K source impedance, NOT at 150 ohm impedance.

> 990 is very similar to a conventional power amplifier

Diff-amp, gain-stage, buffer. That's a VERY useful topology. But it can be implemented MANY ways. Usually you want input device current low for low input DC error. But to come right from 150 or 600 ohm source at lowest hiss you want fairly high current.

The only "benefit" that I see is that you could drive a 8:600 output transformer and get 18dB "free gain", or deliver +/-14V peaks with say 5V supply.

Or you could use +/-25V power to directly drive 10K/8= over one-thousand 10K inputs in parallel. But that's unusual, and normally you'd do mike-amp, gain control, then a Distribution Amp to drive stacks of recorders.

A real disadvantage is that loudspeaker power amps typically can not be set over a wide range of gain. Set gain below 20 they oscillate at MHz. Set gain a lot higher than 20 (say over 100) and the THD numbers look bad.

People often ask if there is no disadvantage doing something that way; I'd rather ask if there is any advantage of doing it that way. Many current IC opamps drive loads below 600 Ohm to +20 dBu with reasonable distortion; and by paralleling or a simple discrete buffer within the feedback loop we easily enhance maximum output current. Now we just need a reason for such high output currents...

Samuel

Thanks

The problem is complex. And the origin is I don’t understand why the 990 module is much much much better than another thing.

I read in some places that the secret is, plus other aspects of course, the MJE181 / MJE171 pair, because it’s bigger than the end of a conventional OPA: “high-power output transistors are fully optimized for they unique and very different requirements” (textual John Hardy datasheets). Well, if I assume that is necessary a big end for “quasi esoteric” aspects, I can use a power IC….!

But I look a 990 schematic and is very similar than a 30 years little power amplifiers, and very similar than IC power amplifiers with differential gain stage front end, (with symmetrical power supply).

OK..., its true…, the power IC datasheets don’t advice les than 20dB gain, and a maximum is no specified (it’s not its work) but I think is 35 or 40 dB maximum for stability. But… I can use another 5534, OPA602 or other AOP step.

And the 990 costs the same than one hundred 5534….

I have a lot of doubts:

1.- ¿Why the 990 is much much much much better than another things?

2.- ¿Why (as I Know, I can be wrong) the most expensive microphone amplifiers use today that technology, (and with input and output transformers)?.

3.- ¿Why the 990 is better than another thinks, and its schematic is similar than a 25 or 30 years old discrete little power amplifiers with differential gain stage front end?.

4.- ¿Why the SSM2019 or derivates are the worst thing in the world.?

5.- Why the cheap is worst and the expensive is good?

Miquel

miquel said:

Thanks

The problem is complex. And the origin is I don’t understand why the 990 module is much much much better than another thing.

I read in some places that the secret is, plus other aspects of course, the MJE181 / MJE171 pair, because it’s bigger than the end of a conventional OPA: “high-power output transistors are fully optimized for they unique and very different requirements” (textual John Hardy datasheets). Well, if I assume that is necessary a big end for “quasi esoteric” aspects, I can use a power IC….!

Click to expand...

Perhaps you need to ask the people who claim it is "much, much, much.....better "

But I look a 990 schematic and is very similar than a 30 years little power amplifiers, and very similar than IC power amplifiers with differential gain stage front end, (with symmetrical power supply).

Click to expand...

not really... The one distinguishing difference about the 990 from other GP amplifiers is the use of inductors in the input long tail pair. This is a clever circuit trick that can't be easily used inside ICs.  That said, that trick may make the 990 better than similar DOA, not better than all modern ICs. However if I were a marketing puke, that's what I would say.

OK..., its true…, the power IC datasheets don’t advice les than 20dB gain, and a maximum is no specified (it’s not its work) but I think is 35 or 40 dB maximum for stability. But… I can use another 5534, OPA602 or other AOP step.

And the 990 costs the same than one hundred 5534….

I have a lot of doubts:

1.- ¿Why the 990 is much much much much better than another things?

Click to expand...

it isn't

2.- ¿Why (as I Know, I can be wrong) the most expensive microphone amplifiers use today that technology, (and with input and output transformers)?.

Click to expand...

fools and their money are easier to separate with a good show and story.

3.- ¿Why the 990 is better than another thinks, and its schematic is similar than a 25 or 30 years old discrete little power amplifiers with differential gain stage front end?.

Click to expand...

but it isn't

4.- ¿Why the SSM2019 or derivates are the worst thing in the world.?

Click to expand...

I am not intimately familiar with 2019 but it probably isn't that bad. Surely not the worst in the world.

5.- Why the cheap is worst and the expensive is good?

Click to expand...

cheap, generally requires use of lower quality components, more expensive does not guarantee anything, only that it could be better, not that it will be.

Miquel

Click to expand...

I don't know if 990 has been benchmarked against modern opamps recently. Back when it was designed a few decades ago the LM394  (actually an IC) input devices was lower noise than available IC opamps. So the old 990 when new, was audibly better than it's IC competition. Times have changed in the decades since then.

JR