Driving an Audio Amp

Not exactly a design question but requires a more knowledgable audience than some other forums out there. Here we go. I have one of these wonderful old Technics stereo amps; massive great thing about the size of an old Betamax video player. Has several inputs at the back: phono, CD, tuner, aux... etc. I want to drive the aux input with my mp3 player, but it's not quite powerful enough so the output volume of the amp is low and slightly noisy. Same problem if I try the CD and tuner inputs. I won't even try doing it with the phono input as even my little player would massively over-drive it possibly causing damage and anyway I believe phono inputs use a RIIA filter to give extra bass boost which would not sound good with any signal other than one from a record turntable. I want to try using sub-min audio transformers (one per channel) between the mp3 player and the aux input to increase the signal levels. Discrete pre-amps of any quality are too expensive so that's why I'm going this route. Anyone see any possible issues by doing this? Will the transformers alter the frequency band of the signals, for example, acting like an unwanted tone control?

thanks

Reply to
Chris
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I find that a bit surprising. Actually, more than a bit, if it's an amp with multiple inputs.

In the pre-CD era, the most common nominal input voltage for line-level preamplifier inputs was typically 1 volt peak-to-peak. It was raised to 2 volts peak-to-peak after CDs arrived.

Audio power amplifiers (those without a preamp and gain stage) do tend to require more voltage to reach full power... they commonly have around 26-27 dB of gain.

Is your amp strictly a power amp? or is it an "integrated" amplifier with a volume control?

Correct. I don't think you'd be likely to damage the amp but it would not sound good.

I suspect that a lot of the audio-type transformers you're likely to find today, are likely to be of "telephony grade" and may not handle the full audio frequency range particularly well... they may result in a non-flat frequency response, and perhaps some distortion as well.

High-quality audio transformers _are_ available, but I have my doubts as to whether you'll like the cost. You might find it less expensive to pick up a decent older-vintage audio receiver or preamp at a local flea market.

Reply to
Dave Platt

I searched for " preamplifier " on aliexpress and found a module using a NE5532 for $3.50 including shipping. You would have to figure out some power for it.

I think the module would be higher quality than a audio transformer.

Dan

Reply to
dcaster

Yes, an mp3 player has relatively low impedance and low voltage output, and might have a level mismatch to your amplifier.

The internal AM/FM, and phono, levels will all work at the usual volume control setting, but for the MP3 player and AUX input, that's too quiet? If that's because of the impedance mismatch, there's three solutions. One, as you say, is an impedance-matching transformer. Those used to be common and inexpensive, now are rare. Second, you can use an external preamplifier with a level control, in series with the power amplifier's volume control, to do the level matching. Third, you can modify the amplifier (build another gain stage INSIDE the big box.

Easiest on my stereo would be to plug a preamplifier into the TAPE MONITOR function, so that the secondary 'tape' input was the new MP3 input. You could run a preamp for a long time on batteries, but you might also be able to sneak a few wires from the amplifier, to provide power for the external box.

There's elaborate prebuilt preamps, based on NE5532, on eBay that would work; just ignore tone controls and use the volume knob. You do NOT want "RIAA" or phonograph or 'phono' preamps, as you know.

If and as the scheme works, it might be moved inside the big roomy box of the amplifier.

If you can play a test tone on your MP3, an AC voltmeter might be employed to determine the voltage gain required. Otherwise, there will be some cut-and-try adjusting to do. The test tone is important, you want to use the FULL RANGE of that MP3 conversion, and not just some random MP3 recording settings.

It's possible, too, to just put an attenuator network into the TAPE MONITOR loop, and toggle it to get your amplifier gain reduced for the inputs OTHER than MP3.

Reply to
whit3rd

meh

fault in the amp? Or do other standard sources drive it fine?

It is possible to reduce the signal and apply anti-RIAA filtering, but pointless extra hassle.

yuck.

Just build a simple amp. It's not hard.

NT

Reply to
tabbypurr

is the volume all the way up on the mp3 player?

m
Reply to
makolber

Chack that the cable is good: perhaps the shield is broken or the sleeve not making contact?

(If the cable is good) that sounds like a good idea to me.

Only if they saturate...

How much is the signal too low by?

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Reply to
Jasen Betts

I can't tell you in dB. I can only say that to my ear, it needs to be approx. 2-4 times louder than it is currently. I believe that would do it.

Reply to
Chris

approx. 2-4 times louder than it is currently. I believe that would do it. "

Twice the loudness requires a 10 dB gain. That is power so it is about 3.16

3 voltage gain.

However, I have heard the have put volume limits on some of those players b ecause they say it causes hearing damage. The setting for that might be in some setup menu or something. I would get the full manual off the net and r ead it. You might just find something.

Reply to
jurb6006

I found nearly a dozen different types of sub-min audio transformers in my junk box so had a good range to choose from. When tested, as expected, some provided better drive to the amp than others. Curiously, there was a tonal variation between the different types with the physically smaller ones producing a sound that was noticably "toppy" or treble-rich. I eventually settled on the largest type I could find (still only the size of a poker dice) since in addition to producing a worthwhile boost in signal level, also sounded the flattest tonally (by which I mean handled the frequency spectrum equally). All these spare trannies looked visually of poor quality; something the Chinese probably produced in the 1980s if you recall the fruits of that era. However, the physics these things exploit is universal, so the fact that they look a bit crappy had no deleterious influence on the sound whatsoever. All in all, entirely successful. I'm just left wondering why the smaller trannies struggled to adequately reproduce the bass frequencies. :-/

Reply to
Chris

Are you serious? Do the math. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

OK, I sort of have. Since the primary of the transformer is in effect a coil, it should function to some extent as a choke - choking off the high frequencies and permitting the lower ones to pass unmolested. This is the opposite of what I would expect! I'm missing something, aren't I? I just have this hunch I'm making a big error here somewhere...

Reply to
Chris

Look up the 'pi-equivalent' for a transformer. Then it'll be clear to you. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

Yes, unmolested to ground!! :-D

Reply to
Cursitor Doom

,
a

y

He can't. It's a civil rights issue.

Reply to
stratus46

[snip]
[smirk :-] ...Jim Thompson
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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

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