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passive audio impedance matching

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scrawny git

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I need to knock up a little circuit that matches the output impedance on a cassette player to the input that is expected by the little power amplifiers I am using.

I believe the impedance of the tape player's output is in the range of 4-8Ω and the amplifier is expecting approx 8.2kΩ.

If anyone's got any pointers to useful circuit designs for this application they'd be much appreciated
 

Nigel Goodwin

Super Moderator
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I need to knock up a little circuit that matches the output impedance on a cassette player to the input that is expected by the little power amplifiers I am using.

I believe the impedance of the tape player's output is in the range of 4-8Ω and the amplifier is expecting approx 8.2kΩ.

If anyone's got any pointers to useful circuit designs for this application they'd be much appreciated
You don't need to match it, and don't want to match it - it's a VERY bad idea - just feed it via a resistor to attenuate it a little (anywhere 10K to 47K should be fine).
 

scrawny git

New Member
well, ok, maybe 'match' was the wrong term but at the moment the power amp barely even notices it's got a signal going into it! Surely I need to at least bring the impedance up into the kΩ?

The speaker output of the tape machine is going through a little 2 channel passive mixer (just across a 47k pot and then through 47k resistor). The output from this works fine if I plug it into a guitar practice amp, but is barely audible when wired through the amp. which by the way is this one:
M034

Is it really enough to just whack a 10kΩ resistor in line (with the signal i assume)?
 

audioguru

Well-Known Member
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Your passive mixer has resistor values that are way too high and make an attenuator.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
well, ok, maybe 'match' was the wrong term but at the moment the power amp barely even notices it's got a signal going into it! Surely I need to at least bring the impedance up into the kΩ?

The speaker output of the tape machine is going through a little 2 channel passive mixer (just across a 47k pot and then through 47k resistor). The output from this works fine if I plug it into a guitar practice amp, but is barely audible when wired through the amp. which by the way is this one:
M034

Is it really enough to just whack a 10kΩ resistor in line (with the signal i assume)?
You never mentioned a passive mixer at all - which is a completely different story.

You need to drastically reduce the values in your crude mixer, try 4K7 instead of 47K for both pots and resistors.
 

scrawny git

New Member
Your passive mixer has resistor values that are way too high and make an attenuator.
The book I got it from said "anything between 10k and 100k as long as the values of the pot and resistor are the same" But i've tried bypassing the mixer and wiring the speaker output straight to the amp, and still no joy. Should I rebuild the mixer with 10k values? or even less?

The signal from the tape machine will drive a small speaker happily so gain can't be the problem otherwise I'd just stick a preamp in between the mixer and the amp.
 

scrawny git

New Member
You never mentioned a passive mixer at all - which is a completely different story.

You need to drastically reduce the values in your crude mixer, try 4K7 instead of 47K for both pots and resistors.
Thanks.

I know they're crude but they are tiny and they do the job...
 

scrawny git

New Member
You need to drastically reduce the values in your crude mixer, try 4K7 instead of 47K for both pots and resistors.
Point taken and I will try this BUT if bypassing the mixer and sending the signal straight to the amp doesn't work then I don't see that this will either...:confused:
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Point taken and I will try this BUT if bypassing the mixer and sending the signal straight to the amp doesn't work then I don't see that this will either...:confused:
No it won't, you hadn't mentioned that previously though.

It sounds like the power amp has a problem?, the spec doesn't mention it's input impedance, but claims the sensitivity is 500mV, which should be plenty direct from the speaker output of the player.

Do you have a scope to test it with?.
 

scrawny git

New Member
ok, apologies for not telling the full story from the word go.

I've tested the amp from another source and it works fine. The spec for the recommended preamp for this unit (M040) say it outputs at 8.2kΩ, so i was assuming that this was a reasonable impedance to aim for inputting to the amp.

As I said above the signal I want to amplify works fine running through a guitar practice amp (even after my crude mixer).

edit: no, I don't have a scope
 
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