1176 best low noise build

[Spencerleehorton]

I know there is a plethora of 1176 threads so I’m happy for this to either be moved or added to another if needed but I have a few mnats pcbs that have taken some damage but I’m interested in breathing new life into them and getting them going with as lower noise as possible.
I’m not really interested in getting the rev A mojo and just wanted this to compress really well and be low noise.
I’m working from the rev F schematic which seems to be the current low noise revision?
The pcbs I have are rev J but have compared the rev F to rev J and there are only a few differences.
Using the LM317 for the psu seems to be better than using the noisy 7824.
Matching the 10k on the input ic option is meant to really help, with 1ohm.
I can compare this as I’m going to make a stereo version inside a 2U.
I’m using the OEP 2:1 output txf and the NE5532 input IC.
So far rebuilding the first pcb I’ve got the preamp and line amp working with correct voltages, meter seems to sort of work but no compression yet so I need to look at the GR circuit next.
Using 2N5457 for the FETs
BC560, 2N3904, 2N5087 for the PNP
BC107, 2N5088 or 2N3391 for NPN
BD139 and BD140 for Q8 and Q9

I’ll list the substitutions I make and post up and other info needed.
Any help is very much appreciated.

 

[Spencerleehorton]

The updated schematic

The old schematic

You should be able to see the differences within the line amp and psu.

 

[Spencerleehorton]

got the first pcb going, there is some hiss so something to work on noise wise, not a whole lotta compression either?
but meter works, GR is showing some compression and +4 works.
only using a cheap VU meter so will try out a few others and see if i can setup this channel like the instructions say.

 

[Spencerleehorton]

Replaced 5532 for a 4558 ic and hiss gone and wiggled a 100uf cap which needed re soldering, now sounds really good, huge output which seems weird and not a great deal of compression?
I’ll start on the other channel and go with the updated schematic rev F and compare once working.
This is what I will change:
R29 from 1M to 6.8M
R30 from 200k to 1M
R32 from 2k to 1k
R33 from 11k to 4.7k
C13 100uf to 2.2nf
R34 from 4.3k to 3.9k
Add 150nf at 17
C27 take out and replace with 220nf in parallel with R7
C9 47uf to 1uf
R21 7.5k to 6.8k
Change psu to LM317 version
Change all 5 x 10k around input IC to match within 1R or better.

 

[Spencerleehorton]

Diode bridge with 1N34A on VU meter and 5k trimmer is needed.
Also need to figure out how to put LEDs inside the VU meters.

 

[Potato Cakes]

I know there is a plethora of 1176 threads so I’m happy for this to either be moved or added to another if needed but I have a few mnats pcbs that have taken some damage but I’m interested in breathing new life into them and getting them going with as lower noise as possible.
I’m not really interested in getting the rev A mojo and just wanted this to compress really well and be low noise.
I’m working from the rev F schematic which seems to be the current low noise revision?
The pcbs I have are rev J but have compared the rev F to rev J and there are only a few differences.
Using the LM317 for the psu seems to be better than using the noisy 7824.
Matching the 10k on the input ic option is meant to really help, with 1ohm.
I can compare this as I’m going to make a stereo version inside a 2U.
I’m using the OEP 2:1 output txf and the NE5532 input IC.
So far rebuilding the first pcb I’ve got the preamp and line amp working with correct voltages, meter seems to sort of work but no compression yet so I need to look at the GR circuit next.
Using 2N5457 for the FETs
BC560, 2N3904, 2N5087 for the PNP
BC107, 2N5088 or 2N3391 for NPN
BD139 and BD140 for Q8 and Q9

I’ll list the substitutions I make and post up and other info needed.
Any help is very much appreciated.

I recently changed out my stereo 1176 Rev D to be a Rev A and Rev D in the same chassis and changed the PSU from a transformer bases linear power supply to two small SMPS units with the capacitance multipliers. Previously, there was always a small hum/buzz when monitoring it at louder volumes. Now I barely get a hiss at those same volumes. If you wanted to reduce noise I would recommend going this route with your PSU.

Thanks!

Paul

 

[gyraf]

Noise in any FET-based simple-potential-divider compressor like the 1176 is probably dominated by the needed attenuation going into the FET - in this case the R6, (27K)

Yes, the -LN mod helps, but we still have this resistor in series with signal, at the input of our amplification stages. The noise contributions of later stages are probably swamped by this

If we decide we can live with less maximum GR, we can get less noise by lowering resistance here

/Jakob E.

 

[Spencerleehorton]

Noise in any FET-based simple-potential-divider compressor like the 1176 is probably dominated by the needed attenuation going into the FET - in this case the R6, (27K)

Yes, the -LN mod helps, but we still have this resistor in series with signal, at the input of our amplification stages. The noise contributions of later stages are probably swamped by this

If we decide we can live with less maximum GR, we can get less noise by lowering resistance here

/Jakob E.

I might try a 25k trimmer with a 2k resistor here at R6 and see what I can get.

 

[dreamix]

I have an rev AB. What noise is it everyone always talk about, i never thought it was noisier than other units in the rack, but maybe im less concerned, or hearing loss

 

[Spencerleehorton]

well im a little stumped at the moment, on the first channel all voltages seem to be well within what they should be, i get clean signal through now, no distortion, on +4 VU meter works on GR meter calibrates but doesnt show any GR and i dont hear any compression on either GR or +4, and when i turn the meter control to off the sound doesnt change?
when i twiddle the release there is signs of the needle moving slightly
need to find out why ive not got compresion!!
Im using both J201 fets for Q1 and Q10, ive double checked pinout and connections on pcb and all is correct?
when adjusting Q bias either way it only changes the meter needle, no difference in sound?

any help would be fantastic!!