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microcassette preamplifier hiss

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neazoi

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Hi this little microcassette preamplifier worked ok but it has much hiss.
I need it to record only 300Hz-3KHz (HF radio) so high frequency response is not needed.
What can I do to remove the hiss?
Any ideas to try?
 

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I can still remember old micro-cassette tape recorder/players.
You show only the playback circuit but maybe the record circuit also produces hiss. Maybe the brand of tape is a cheapy.
A TL071 has a typical noise of 18 thingies. An NE5534 has a typical noise of only 3.5 thingies. Much less noise and its pins are the same so nothing needs to be changed to use it.
 
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    neazoi

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A TL071 has a typical noise of 18 thingies. An NE5534 has a typical noise of only 3.5 thingies. Much less noise and its pins are the same so nothing needs to be changed to use it.

I tried the NE5534 but not a big deal. I left it there just because of your recommendation, but the hiss is still there. I think that a kind of LPF at some point (between the first and second opamp?) might do it, since I only need up to 3KHz.
Maybe alter one of the two stages to behave as a LPF? How?
Any ideas of how to remove this hiss would be appreciated.
 

I forgot that little tape players normally produced hiss which is why Dolby Noise Reduction was used.
Your circuit already cuts all the high frequencies in music and all consonants in speech to make very muffled sounds. The two 220pF capacitors and the 180k resistors are lowpass filters that cut frequencies above 4kHz each. Then the response drops above 2khz.
The NE5534 opamps do not work properly with filters like that so put back the TL071 opamps.
 
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    neazoi

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Hi,

If very low voltage input signal, unlikely to work but could try to reduce resistors by an order of magnitude or more, thinking of Johnson noise, see Op Amps for Everyone. "Thermal noise is amplified by gain of circuit."

Are capacitors right sort/dielectric for that kind of circuit?

Can you measure or infer frequency of hiss? If so, band-pass or band-stop filter could work to reduce it. Nuisance of high Q active and passive filters is number of stages required to get steep responses, not slow roll-off as with first-order filters. Better filters = more stages = more components...

Really recommend you take a peek at Op Amps for Everyone chapters on filters and chapter about resistors used with op amps regarding noise and Vio Iio, etc.
 

Low-pass active filter. It's second order due to having two capacitors. I believe it's Sallen-key type, reputed to do the job with low parts count.

low pass op amp 2 caps 3kHz cutoff (sallen key).png

Signal from tape head is very low amplitude. By finagling with resistor values you can achieve higher gain. However it's tricky to try to do everything in one stage. Sometimes it's better to to build a second stage for amplification before or after the filter stage.
 
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What does crossed out R4 in the post #1 schematic mean? Removing this resistor is a really bad idea, creating a strange frequency response and possibly oscillations.
 

What does crossed out R4 in the post #1 schematic mean? Removing this resistor is a really bad idea, creating a strange frequency response and possibly oscillations.
the only think it did there was to reduce the signal out of the first opamp, so requiring a third opamp for further amplification. I do not think the hiss is considered as some hind of high frequency oscillations ?

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Low-pass active filter. It's second order due to having two capacitors. I believe it's Sallen-key type, reputed to do the job with low parts count.

View attachment 158699

Signal from tape head is very low amplitude. By finagling with resistor values you can achieve higher gain. However it's tricky to try to do everything in one stage. Sometimes it's better to to build a second stage for amplification before or after the filter stage.

Allright, I will try altering the second stage with this one.
 

the only think it did there was to reduce the signal out of the first opamp, so requiring a third opamp for further amplification.
R4 is required to set a useful gain in inverting OP configuration. Why are you ignoring the basics of OP circuit design?
 

R4 is required to set a useful gain in inverting OP configuration. Why are you ignoring the basics of OP circuit design?

I do not know about opamps really... I avoid them when I can in favor of discretes.
Without R4 there is much more gain though!

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The two 220pF capacitors and the 180k resistors are lowpass filters that cut frequencies above 4kHz each. Then the response drops above 2khz.
The NE5534 opamps do not work properly with filters like that so put back the TL071 opamps.

Inspired by this, I replaced the two 220pF in both opamps with 1nF ones. The noise was reduced 80-90% without reducing the AF volume (well just a bit).

And yes it works fine with NE5534 as well.
 

More gain yes, but a completely different frequency characteristic.

amp.PNG
 
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    neazoi

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More gain yes, but a completely different frequency characteristic.

View attachment 158707

Thanks for your time in putting this here!
It seems about 17dB lower at 1KHz with the resistor in place. On the other hand without the resistor it peaks to 4KHz.
I think that the change in the capacitors I did above, did barely reduce the gain at lower frequencies (by ear) but was very effective in cutting the hiss and highs. I am very happy with the tested result really, without extra components or opamps.
 

I remember now that when I played with cassette units 56 years ago that they used pre-emphasis and de-emphasis equalization like phono players (remember them?) and FM radio.
During recording, the low frequencies were cut and the high frequencies were boosted. Then the reverse during playback the low frequencies were boosted back to normal and the high frequencies were cut and the hiss produced by the tape particles was also cut.
 

I remember now that when I played with cassette units 56 years ago that they used pre-emphasis and de-emphasis equalization like phono players (remember them?) and FM radio.
During recording, the low frequencies were cut and the high frequencies were boosted. Then the reverse during playback the low frequencies were boosted back to normal and the high frequencies were cut and the hiss produced by the tape particles was also cut.

It is called Dolby-B. In this comms system the opposite is needed. Cut the hiss (very highs) during recording (I will use the same preamp as the play one) and cut the highs also on play. This play/record EQ will make a sharper cut off of the highs.
 

Are you making an underwater throat mic effect where nobody can understand what is being said because all the important high frequency consonant sounds in speech are missing? Then you need an extra button that the user can push for a recording to say, "What did you say?".
 

Pre-emphasis and de-emphasis is not the same as Dolby-B! Mr Dolby would get very upset if you said so :lol:
Dolby systems use a sliding emphasis depending on the audio amplitude and a non-linear amplitude characteristic.

Brian.
 

I agree that all audio tape recorder/players used NAB equalization. CDs and MP-3 replaced tape. I have a portable Kenwood CD player. My Creative portable MP-3 player has a mechanical hard drive in it!
I haven't used my Yamaha cassette deck for at least 25 years. It has Dolby C and chromium tape for a hifi response.
 

Here is the diagram of the tape recorder/player up to now. It is not complete and I draw it as I build it.

I have a weird hum to it's output which is due to the motor. When I disconnect the head from the preamplifier no hum on play mode. When I connect it there is this low hum.
Depended on the speed of the motor, the hum raises in frequency or lowers.
The head body is grounded.
I need an expert audio guru here!
 

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Hi,

Now it's "hum", before it was "hiss". Is it the same problem or another problem?
(I'd put "hiss" in high frequencies, while "hum" is low frequency)

Hum from motor:
I'd say it's not a schematic problem, but a wiring problem.
--> Use star ground wiring, avoid ground loops.

Klaus
 

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