Driving an Audio Amp

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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

Re: Driving an Audio Amp
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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?

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

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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.

Re: Driving an Audio Amp
On Monday, July 10, 2017 at 3:04:27 PM UTC-4, Chris wrote:
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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


Re: Driving an Audio Amp
On Monday, July 10, 2017 at 12:04:27 PM UTC-7, Chris wrote:
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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.

Re: Driving an Audio Amp

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

m

Re: Driving an Audio Amp
On Monday, 10 July 2017 20:04:27 UTC+1, Chris  wrote:

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meh


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

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

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yuck.

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


NT

Re: Driving an Audio Amp

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

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

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Only if they saturate...

How much is the signal too low by?



--  
This email has not been checked by half-arsed antivirus software  

Re: Driving an Audio Amp
On Tue, 18 Jul 2017 06:03:50 +0000, Jasen Betts wrote:

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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.

Re: Driving an Audio Amp
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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.

Re: Driving an Audio Amp
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. :-/




Re: Driving an Audio Amp
wrote:

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Are you serious?  Do the math.
        
                                        ...Jim Thompson
--  
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
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Re: Driving an Audio Amp
On Sun, 23 Jul 2017 09:07:40 -0700, Jim Thompson wrote:

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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...

Re: Driving an Audio Amp
wrote:

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Look up the 'pi-equivalent' for a transformer.  Then it'll be clear to
you.
        
                                        ...Jim Thompson
--  
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
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Re: Driving an Audio Amp
On Sun, 23 Jul 2017 16:21:29 +0000, Chris wrote:
  
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Yes, unmolested to ground!! :-D

Re: Driving an Audio Amp
On Sunday, July 23, 2017 at 9:07:54 AM UTC-7, Jim Thompson wrote:
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,  
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a  
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y  
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He can't. It's a civil rights issue.


Re: Driving an Audio Amp
On Sun, 23 Jul 2017 12:46:07 -0700 (PDT), stratus46

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[snip]
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[smirk :-]
        
                                        ...Jim Thompson
--  
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
We've slightly trimmed the long signature. Click to see the full one.

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