reviewing a preamp design

[Audioguru]

Your preamp should work fine with line level input audio.

 

[Enzy]

Ok Ill make that change tomorrow and let you know how it sounds.

 

[Enzy]

I made the changes but was still getting noise, I removed the boards and re-soldered some joints and realized it was some open circuits causing the noise one 1 amplifier.

The other amplifier is still noisy If I remove the 50k Pot its silent, if I put it back in and turn it up then it get noisy, I changed pots and its the same thing. If I turn it all the way up I get like 1.7v from speaker output of the amp. What d you think is causing that.

 

[Enzy]

I made changes to the amp using the 1M and a 33k resistors and it sounded louder, I placed back the 300k and 10k resistors and It was lower so I paralleled a 300k resistor to the existing 300k resistor and the noise went down but at less than half way turning the 50k nob up it becomes too evident in the music that its noisy.

when no music is playing after I parallel the 2 300k resistors I got 0.7v maximum so It reduced the noise but it still needs to go lower.

 

[Audioguru]

With 1M and 33k resistors the gain is 1+ (1M/33k)= 31.3 times and with 300k and 10k resistors the gain is 1= (300k/10k)= 31 times which is a negligible difference. Maybe your resistors are cheap ebay quality?
Please describe the noise. High frequency hiss or low frequency mains hum? Maybe your opamp is counterfeit? Replace it with one purchased in USA.

 

[Enzy]

actually my aim was to bring the gain down to 11 like you said wasnt that why you suggested using 1M and 30k resister?

What I did so far was to swap the opamp with the side that works well and it sounds the same when, I have about 3 others also and they still make the sound when I try them.

I think I would have to record the sound of the amp to let you hear it.

 

[Audioguru]

In post #39 I said, "For a gain of 11 times then use 300k and 30k resistor, and a 330nF or 470nF film capacitor." But you used 1M and 30K for a gain of 34.3 times. The increased gain is not much louder, simply turn down the input volume control a little.
For a gain of 11 times the resistors can be 100k and 10k and also use the 330nF capacitor.

Since you cannot describe the noise in words then please record it.

- - - Updated - - -

On your pcb there is a 470k resistor and a ceramic capacitor that might cause trouble:

 

[Enzy]

I'm no longer using the 470k I changed it to 330k but to test the theory I added another 330k resistor across the one on the board which should divide the resistance in half it reduced the hum but it's still there.

The ceramic capacitor I used is the 1uf capacitor on the output, I only had 1 the other one which I used was a electrolytic capacitor, that's the side which plays with the most hiss/hum not sure which it is.

I'll send recording link soon

 

[Audioguru]

Thank you, you finally describe the "noise" as hum.
You have many inches (centimeters) of pcb tracks acting as antennas and picking up the mains hum from your AC wiring all over the project. You even have high voltage AC wiring running closely past the preamp pcb and low voltage AC on the preamp pcb. AC and audio should be completely separate.
The 100 ohms output resistor is on one end of the huge pcb and the 1uF output capacitor is on the other end. But the input wires and feedback pcb tracks pick up the most hum.
Please post the actual schematic because the existing schematic does not show a 470k resistor so I do not know which resistor it is in the circuit. Usually a schematic is marked "R1, R2, R3... and C1, C2, C3..." then the parts on the pcb are marked the same. Can you mark them please?

 

[Enzy]

I'm at work now I'll post later when I'm home, I actually looked back at the circuit this morning and realized that I still had the 470k in the circuit and im sure its not suppose to be there I'll correct that when I'm home and I really need to plan out how to run my wires and also decrease the length of the traces

 

[Enzy]

also I described it as a hum this time because it finally had a familiar sound, the other times it sounded like noise so wasnt sure if that was a hiss or a hum

- - - Updated - - -



this is the actual schematic that I am using, I have the 4 diodes on my (rectifier stage with a cap to filter) but I removed it since im making a much smaller board now and Ill build a single power board to supply all boards in my amplifiers next time around so that I can route all wires easier.

 

[Enzy]

Ok ill be doing my next project in a bit, using the class D amplifier here is a link to the board
https://www.ebay.com/itm/182307479849?_trksid=p2057872.m2749.l2649&ssPageName=STRK:MEBIDX:IT

- Power: 300W 8R (+ -75V), 600W 4R (+ -75V), 1000W 2R(+ -75V)
- Power supply voltage: DC+-60V~+-80V, (recommend voltage DC+ -75V)
- Distortion:0.07 % THD+N 600W 2 R
- Voltage gain:33 DB
- Loading resistance: 2Ohm
- Weight:180g
- Size: 129MM*72MM*43MM
- Total Idle Power Consumption: 13.2W
- System Efficiency: 80% 600W 2R
- Dynamic Range : 99.4 DB
- Residual Noise: 290 UV
- Damping Factor:>70 (1K HZ ,2R)
- Frequency Response: 20Hz-20kHz + -1DB
- Thermal Performance temperature: Idling <60C
125W <104C
850W 2R< 118C (If always loading, it is better to add fan)

with a gain of 33 what kinda gain would be recommended from a preamp.

 

[KlausST]

Hi,

33dB is not gain of 45.

If your output voltage is +/-75V, then divide this by the gain of 45 and get an input voltage of +/-1.7V.

Now adjust your preamp output level to this value.

Klaus

 

[Audioguru]

With low distortion the amplifier produces 600W into 2 ohms so its output is 35V RMS. Its gain is 33dB which is 45 times. Then its input is 35V/45= 778mV RMS. A consumer audio product produces about 316mV so additional gain of 778/316= 2.5 times is needed and about 3 times more for low levels to be turned up. Then the preamp gain should be 2.5 x 3= 7.5 times.

 

[Enzy]

Ok That means I could use the schematic with the following parts which seems to be about 7.6 times gain



so 600w*2=1200
then find the square root= 35v

so most laptops and would produce up to 316mV with low distortion? I have herd you mention 316mV so many times but now im actually just understanding how its used.

- - - Updated - - -

but tell me something else if what I said so far is correct, should I use preamp to cut frequency for the amplifier or should I allow the preamp to send high and low frequencies and then build an an active cross over separately to do that?

 

[Audioguru]

Ok That means I could use the schematic with the following parts which seems to be about 7.6 times gain



so 600w*2=1200
then find the square root= 35v
I forgot that you are using simple stereo to mono resistors that cut the signal level a little so increase the 68k to about 100k.

so most laptops and would produce up to 316mV with low distortion?

It is a standard but all recordings and laptops do not follow it. Some are higher and others are lower. Gain does not need to be accurate because you have a volume control to adjust levels.

Should I use preamp to cut frequency for the amplifier or should I allow the preamp to send high and low frequencies and then build an an active cross over separately to do that?

A crossover uses active filters so that the cutoff is sharp. The RCs in the preamp create a gradual cutoff that is poorly defined then you have lows and mids or mids and highs adding and producing a peak in the frequency response. The phase at cutoff is also important to avoid adding levels or cancellation at the crossover frequency. The lowpass cutoff filter must match the highpass filter feeding the mids. The lowpass filter feeding the mids must match the highpass filter of the highs.

 

[Enzy]

another thing I need to clear up I normal use online calculators to check cut off frequencies but they dont seem accurate to me. for example in the preamp at the output the 1uf cap and the 50k pot would make and RC high pass filter but in the calculator it says 3hz, so is that saying it will allow anything about 3hz?

so in other words it can play all frequencies we can hear.

 

[KlausST]

Hi,

50k, 1uF

fc = 1 / (2 x Pi * R * C) = 1 / (2 x 3.14 x 50k x 1u) = 3.18 Hz.

but they dont seem accurate to me

What else did you expect?

Klaus

 

[Audioguru]

A highpass RC of 50k and 1uF produce a reduction in level of -3dB at about 3.2Hz, -6dB at 1.5Hz and -12dB at 0.75Hz. This single RC starts cutting at about 15Hz. Another simple RC causes the same but your preamp has 3 of these RC highpass filters. Then 3.2Hz is reduced to -12dB but the rolloff is so gradual that 32Hz is also reduced but not as much. You probably want 20Hz to be flat without any reduction.

 

[Enzy]

Hi,

50k, 1uF

fc = 1 / (2 x Pi * R * C) = 1 / (2 x 3.14 x 50k x 1u) = 3.18 Hz.

What else did you expect?

Klaus

If you realize I used a question mark, its a question.... So it is saying if the frequency will be higher than 3hz or lower than 3hz.

Nothing to expect more than an honest response sir.

- - - Updated - - -

You probably want 20Hz to be flat without any reduction.

Ill be using this amplifier to play bass so I'm looking at something between 20hz to 150hz since I think that's the range for bass but I didn't want to Ommit any signals coming from the preamp to the amplifier input

- - - Updated - - -

Meaning I want the full bass to tweeter frequency from the preamp output then do the rest with a cross overs.