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Old 1st Jan 2016, 6:01 pm   #1
tin0gauge
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Default Impedance / level matching phono to cassette head

Happy New Year to all those avid electronics enthusiasts.

I am fitting a DAB radio module, an old Matsui unit that was designed to plug into the phono sockets of ones ageing stereo and had the advantage of a remote control. I am connecting the phono outputs to what was the input from the cassette player head in an old stereo system so that when you switch to "cassette" you get "DAB". The level / matching is wrong and overloads the input when the stereo is set to "cassette" I assume because this was designed for a low impedance coil in the cassette pick up head, can anyone please suggest some impedance / level matching components to put in line with the input. The amp is too small and integrated to be able to get onto the main mother board so hoping perhaps a resistor network may do the trick ?? it works OK but the DAB is set to min volume to make it work. thanks Bryan.
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Old 1st Jan 2016, 6:45 pm   #2
dave walsh
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Default Re: Impedance / level matching phono to cassette head

"Sounds" an easier problem than lack of gain anyway! I've just experimented with pots or fixed resistors in the past on a "suck it and See " basis. If the output you get with the DAB at minimum is distortion free the equalisation may not be a significant factor. I'm sure you will get a more sophisicated response from others though
Dave W
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Old 1st Jan 2016, 6:57 pm   #3
Lucien Nunes
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Default Re: Impedance / level matching phono to cassette head

On every solid-state integrated amp I've seen with phono sockets for a tape deck, the input is normal line-level, typically -10dBV into 10k. Only a 'tape control unit' with equalisation, bias and record level controls and the associated circuitry would be intended to connect to a tape head itself, as was common in the days of open-reel decks purchased separately to the electronics. Thus, it's not clear to me why you should experience overload. You're not accidentally going into the pickup input, due to groping around the back of the unit blind, by any chance? I had a situation where I had done just that and due to crosstalk the signal seemed to appear on the CD input too, which was where I had been aiming the plugs in the first place.

E2A, perhaps I misunderstood. Do you mean you have disconnected a head from the replay amp and want to connect the DAB output in its place?
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Old 1st Jan 2016, 7:24 pm   #4
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Default Re: Impedance / level matching phono to cassette head

Thank you good info, yes I have cut the audio wires directly off the playback head on the cassette deck. So this is the input to my stereo amp when switched to cassette. if I was to fit say a 500k a pot in the line to reduce level, which would be the best way round with the pot ? across the audio ( phono) output and the wiper on the audio input or the opposite? if the opposite then the rather sensitive input would then be loaded across the two ends of the pot which may be better ?
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Old 1st Jan 2016, 7:37 pm   #5
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Question Re: Impedance / level matching phono to cassette head

Quote:
Originally Posted by Lucien Nunes View Post
You're not accidentally going into the pickup input, due to groping around the back of the unit blind, by any chance?
By my understanding of what the OP has written, if he had done that, then there would not be a problem, surely, since he has said that "I am fitting a module that was designed to plug into the phono sockets of a stereo (amplifier)", and those phono input sockets would, per se, be the pick-up inputs?
However, the OP has said that he is using the amp. inputs designed to receive signals from a cassette player head. So on that basis, perhaps the OP's need would be easily satisfied by actually feeding the outputs of the module into the pick-up / phono sockets of the amplifier. If, however, the existing pick-up / phono inputs are required to be retained, then perhaps a simple 2-input mixer cct. at the amp. phono inputs would suffice.

As for feeding into inputs that were designed to take signals direct from a cassette playback head, is the frequency response of such input likely to be incompatible with that required for the added module, since that module is designed to see a pick-up / phono-type source, with those particular frequency response characteristics?

Just a few thoughts.

Al.

Last edited by Skywave; 1st Jan 2016 at 7:43 pm.
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Old 1st Jan 2016, 8:05 pm   #6
tin0gauge
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Default Re: Impedance / level matching phono to cassette head

Thank you gentlemen all good input, to make my situation more simple to explain I did say a stereo, however it is not a conventional stereo with nice phono input sockets, everything is hard wired in a very compact unit so wires everywhere and of course no circuit diagram, hence the need to try and use what is accessible to connect the DAB module to the main amp via the Cassette setting on the main unit switch and the wires originally going to the cassette deck playback head. I should have explained myself better sorry. Re the pot use as an attenuator and my previous message which would the best way round to put it ?
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Old 1st Jan 2016, 8:08 pm   #7
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Default Re: Impedance / level matching phono to cassette head

And here is a photo of the beast just to make it all clear, not really a stereo per se.
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Old 1st Jan 2016, 10:28 pm   #8
AndiiT
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Default Re: Impedance / level matching phono to cassette head

Hi there is a circuit here which should reduce the output level of your DAB module to a suitable level for feeding to what is essentially the "tape head" input of your unit.

The circuit is actually designed to tailor the response of a linear signal to allow it to be fed to the RIAA equalised input (phono/disc) of a separate amplifier but should probably suffice for your purpose.

You might have to experiment with the 470 ohm resistor R4 to get the correct level of volume.

I just noticed that your intention is to switch the cassette heads and the DAB module, as the signal levels at the cassette heads is very low use decent screened cable for all your connections due to the high gain of the amplifier at its input otherwise you may experience buzz/hum.

Regards

Andrew
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Old 2nd Jan 2016, 1:28 pm   #9
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Default Re: Impedance / level matching phono to cassette head

Thank you Andrew working on that circuit right away.
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Old 2nd Jan 2016, 2:27 pm   #10
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Default Re: Impedance / level matching phono to cassette head

If you have taken the wires actually off the tape head and you are trying to feed a line level signal from a tuner unit to this point you have two problems: 1) huge difference in supplied level and 'expected' level from the amplifier - by around 200 times
2) equalisation; the output from a tape head is frequency and tape type dependent - the tuner output is essentially flat.

I would try and find in 'insert' point further up the amplifier chain; attempting to feed a signal in at this point will always suffer from poorer signal/noise ratio and most likely slightly compromised frequency reponse. You would have done better leaving the cassette heads alone and using a dummy cassette adapter and feeding your tuner into this; they're common enough, often used to plug i-pods etc into old in-car radio/cassette machines.
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Old 4th Jan 2016, 5:30 pm   #11
jamesperrett
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Default Re: Impedance / level matching phono to cassette head

Might be easier to find one of those cassette adaptors that consist of a tape head inside a cassette shell with a wire coming out of it to connect to a CD player or similar. I have a Panasonic one that works surprisingly well.
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Old 5th Jan 2016, 1:21 am   #12
Nicklyons2
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Default Re: Impedance / level matching phono to cassette head

Quote:
Originally Posted by Nicklyons2 View Post
...... You would have done better leaving the cassette heads alone and using a dummy cassette adapter and feeding your tuner into this; they're common enough, often used to plug i-pods etc into old in-car radio/cassette machines.
Quote:
Originally Posted by jamesperrett View Post
Might be easier to find one of those cassette adaptors that consist of a tape head inside a cassette shell with a wire coming out of it to connect to a CD player or similar. I have a Panasonic one that works surprisingly well.
I wish I'd have thought of that!
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Old 5th Jan 2016, 6:45 pm   #13
jamesperrett
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Default Re: Impedance / level matching phono to cassette head

Sorry Nick - I missed your last sentence
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