reviewing a preamp design

[KlausST]

Hi,

There is a misunderstanding.
My "what else did you expect" did not refer to our answers.
It referred to your sentence: "but they dont seem accurate to me"

To me it seemed you did not expect the "3Hz" but any other value.

Klaus

 

[Audioguru]

Since you want the preamp to produce 20Hz without any loss of level and the preamp has three RC highpass filters then each filter should have a -3dB cutoff at about 1.5Hz. Then -9dB at 1.5Hz, -6dB at 3Hz, -3db at 6Hz and beginning to reduce levels at about 20Hz.

 

[Enzy]

Hi,

There is a misunderstanding.
My "what else did you expect" did not refer to our answers.
It referred to your sentence: "but they dont seem accurate to me"

To me it seemed you did not expect the "3Hz" but any other value.

Klaus

The problem is because I was just watching the values audio guru used and didn't always try to do everything myself also so I was under the impression that the filter at the output would only allow 1khz up and blow lower frequencies, but how it is now it can play all frequencies.

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Since you want the preamp to produce 20Hz without any loss of level and the preamp has three RC highpass filters then each filter should have a -3dB cutoff at about 1.5Hz. Then -9dB at 1.5Hz, -6dB at 3Hz, -3db at 6Hz and beginning to reduce levels at about 20Hz.

I'm yet to understand those terms in your explanation.

But as the preamp is now are you saying it would have difficulty playing low frequencies?

 

[Audioguru]

I calculated the cutoff frequencies of the highpass filters in your latest schematic:

1) The input RC is 330nF into 100k ohms which is a very simple highpass filter. It has a cutoff frequency of 4.9Hz that has the level reduced -3dB. The single RC rolls off lower frequencies at -6dB per octave so 2.5Hz is at -6dB and 1.25Hz is at -12dB. It begins reducing frequencies below about 20Hz.

2) The feedback RC is 3.3uF and 10k ohms that is also a highpass filter. It also has a -3dB cutoff frequency of 4.9Hz.

3) The output RC is 1uF into 50k ohms that is also a highpass filter. It has a -3dB cutoff frequency of 3.2Hz.

The three cutoffs combine and cause low frequencies to be gradually reduced. If the capacitors or resistors have their values increased so that their cutoff frequencies are 3.2Hz then you will have 3.2Hz reduced three times -3dB which is -9dB. The response will begin dropping at about 43Hz. For the response to not have any reduction down to 20Hz then each of the three RC filters must be calculated for a cutoff frequency of 1.5Hz.

These are highpass filters that have no affect at higher frequencies that are passed by the entire preamp circuit.

 

[Enzy]

Ok based on what you said I made some changes.

 

[Audioguru]

I mentioned earlier that the stereo to mono resistors reduce the input level so R2 should be increased to 100k.

 

[Enzy]

ok so the gain would be a bit below 11.

 

[Enzy]

I was missing in action due to Internet issues, I made a pcb for the board design thinking everything was ok with it. But after assembling parts I'm not getting any audio output from the preamp, no voltage.

I tend to be having the same issue everytime, I check a traces and all connections are made and all components are correct except one POT. R9 20k pot I'm using a 50k POT instead because I'm awaiting the correct value, but would that cause me not to hear anything at all and get no voltage out.

 

[Audioguru]

With a high input resistance digital voltmeter measure the DC voltages:
10 the supply is =12V.
The input pin 3 and output pin 6 of the opamp mare at +6V.
Tell us if the voltages are different.

What is your input signal from? The gain is much too low for a microphone.
What does the output of this circuit feed? it cannot drive a speaker, it must drive a power amplifier.

 

[Enzy]

Testing across pins 3 and 6 the voltage is 3.43v dc.

Between Gnd and pin 3 V= 5.4v
Between Gnd and pin 6 V= 1.6v

Supply voltage to opamp is 12.22v

Its supplying a power amplifier.

I also did tests without connecting to power amplifier and just used my volt meter across preamp output.

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My signal is from a laptop

 

[Audioguru]

The DC voltage on the output pin 6 of your TL071 opamp is much too low then it cannot swing up and down. Maybe its pin 1 or pin 5 is shorted to something? Maybe the opamp is a cheap Chinese ebay fake?

 

[Enzy]

OK then I ordered 50 of the Tl071 chips, I have only one that has a different letter at the end.

The odd one works smh.....

I can't beleive that has been my problem all this time, I made so many boards!

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It seems my 50k pot isn't all the good either, can I replace it with a resistor.

Or would it be ok to not use any resistor at all since u already have 10k resistors at the input for the stereo to mono converter.

So I would remove the pot and the line coming from 10k would go straight to 330nf capacitor.

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I tried it and it worked good, the volume is now high. Since the 20k pot and the 330nf cap made a cut off I guess it's not the 10k from the converter along with the cap which will make the cut off range now then

 

[FvM]

The schematic can't explain the low voltage measured at pin 6. Either the OP is defective, or the real circuit is different from the schematic.

OK then I ordered 50 of the Tl071 chips, I have only one that has a different letter at the end.

The odd one works

There are no different TL071 types manufactured. Fake ICs from dubious sources?

 

[Enzy]

The schematic is done in Eagle cad so the board exactly matches the schematic.

When measuring between pin 6 and gnd should I get 6v without signal being applied to the preamp?

I was using a tl071cp I have several of those and I tried about 6 of them which resulted in noise or no sound but when I tried to tl071cn it worked good with no noise.

 

[FvM]

As far as I know is CN "lead-free" (higher solder temperature), CP previous device version. There should be no electrical differences.

 

[Audioguru]

The defective 20k pot will not cause the DC output voltage from the opamp to be too low.
Which company made your TL071 ICs? Years ago, Toshiba made an "improved" one. It oscillated badly at a high frequency and was discontinued but there are probably many sold cheap on ebay.

 

[Enzy]

When I get home I would have to check but I have alot of the 'CP' model chips and all of them sound distorted or getting no sound and I varied the voltage level from 5 to 14v and its the samething.

 

[Audioguru]

I had tens of thousands of an equalizer made with Motorola/ON Semi TL074CP opamps bought from Digikey and they only ones with a problem was one with its IC mounted backwards and another with a bad electrolytic capacitor.
I have used many TL072CP dual opamps made by Texas Instruments and bought from Digikey in many products and they all worked well.

 

[Enzy]

Ill make another check again, if anything what I'll try to do is record it and send the link so you can view hopefully the audio is good enough