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Author Topic: Noob with a soldering iron  (Read 29592 times)

Jungle-Jim

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Re: Noob with a soldering iron
« Reply #75 on: February 02, 2015, 07:36:15 AM »
Thanks again Phil

I'll try those resistor changes later this evening - will report back.
Much appreciated.
Jim

Jungle-Jim

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Re: Noob with a soldering iron
« Reply #76 on: February 09, 2015, 11:20:27 AM »
Hi all
Sorry - another week delay.

I've done the two tests set for me -
Roly - the 0.1uF across pin 4 & 8 of IC3. I noticed that there's + and - rails which links pins 4 and 8 of all the ICs, so the cap could have gone anywhere along that line, but I put the cap very close to IC3 incase that had an impact on noise. Result - no discernible change. I have left the cap on, figuring it can't harm it.

Secondly, I did Phatt's two resistor changes around IC3. A 1k res became 10k, and the 560k outputting the Ch B pre-amp became a 100k. Unfortunately no change. If anything, the mV readings at the speaker changed - they changed to 0.7mV - 2.0mV when the Ch B vol went from 0-10, where as before, and in most other configurations, this was typically 0.7mV - 1.5mV.

I have left the cap in, and I've left the replacement resistors in for the time being, it's a struggle to change components on this board without the pads disintegrating.

This time I have uploaded an audio file - unfortunately recorded with my phone, I simply can't get any recording gear and the amp in the same room conveniently. There is something you can hear there. There is a very slight flanging sound in this audio which isn't in the real thing.

Thanks again for your patience,
Jim

Roly

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Re: Noob with a soldering iron
« Reply #77 on: February 09, 2015, 05:55:21 PM »
Quote from: Jungle-Jim
the 0.1uF across pin 4 & 8 of IC3. I noticed that there's + and - rails which links pins 4 and 8 of all the ICs, so the cap could have gone anywhere along that line, but I put the cap very close to IC3 incase that had an impact on noise. Result - no discernible change. I have left the cap on, figuring it can't harm it.

I was explicit without explaining why.  Just considering DC the cap could go anywhere between the IC and the power supply, but considering AC as you move from the IC pins back along the supply rails you start introducing the series inductance of the PCB traces, and this makes the cap less effective as an AC bypass.

So when I say "right on the IC pins, VHF-style (short leads)" I really mean it.  If the noise were due to a parasitic RF or VHF oscillation then the location of the bypass cap would be critical, but as you have put it on the IC pins and it has made no change I think we can now rule out RF instability as the cause.  Narrows down the field.   :tu:
If you say theory and practice don't agree you haven't applied enough theory.

phatt

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Re: Noob with a soldering iron
« Reply #78 on: February 10, 2015, 06:37:16 AM »
After hearing the hiss,, a couple of observations.

Are you aware that most guitar circuits do have some background noise and by the sound of that small clip you are ahead of most.
If you are playing at low volumes a lot the hiss can become annoying.

Judging by the level of your voice and then the level of hiss which was only evident at the very last I'd say you have no real issue.  8)

Gee I had a chap bring me a Laney Valve Amp a couple of years back and the hot channel was so bad the hiss/buzz/hum was louder than anything less than a power chord. :duh
Frankly I don't think you can do much better than what you have.

Some options 4 U, Turn up the gain at the first stage and then back off the gain after tone section.
As there is a lot of bass cut in the first section this does tend to accentuate the top end and that can make for a fizzy amp.

A 50~100pF cap across that 1Meg FB resistor on IC3 (mixer) will kill a lot of fizz but will effect the tone. In my experience if you want the sweeter sounds limiting the higher freq makes the amp more playable. Just my 2 cents worth. xP

Phil.

J M Fahey

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Re: Noob with a soldering iron
« Reply #79 on: February 11, 2015, 04:14:52 AM »
Listened to the MP3 and it sounds like you speak straight into the microphone and then place it  close to the speaker.

That's unreal, people will not listen to your amp with their heads stuck into the cone.

For a realistic test, please put the microphone 1 meter away from the speaker, your head also 1 meter away , start recording, speak and make silence for at least 15 seconds, then repeat 3 or 4 times, so we can "calibrate" our ears.

That way we can compare hiss/buzz/whatever 1 meter away with a standard voice same distance.

Otherwise we have absolutely no reference.

Do NOT fall in the trap of putting the microphone closer to the speaker "to show us how bad it is" .

Even so, hiss will be exaggerated, because very probably your recording compresses audio, so high level (your voice)  gets attenuated, and low level (hiss)  gets amplified, making it look worse (on recording)  than it actually is.

Thanks.

Jungle-Jim

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Re: Noob with a soldering iron
« Reply #80 on: February 11, 2015, 07:23:49 AM »
Thanks guys
I will have a think what to do with this amp. I have learnt an enormous amount from working on it, but perhaps it's just a noisy amp. I have other SS amps around, and I'm telling you this one has an annoying hiss. It has been a 'project' - with the fantastic help of you guys - to see if we could quieten this amp. I guess if I got it to a technician maybe the source of the hiss could be found, maybe he'd just say it's a hissy amp.

Criticisms of my recording - unfortunately I could only use my phone, so that's not a great start. Yes I put the phone near the amp, but there's already noise and hiss from the phone, so there was no point in having the phone at a distance where you couldn't separate it's own hiss from the hiss of the amp. Believe me from one metre the hiss on this amp is noticeable.

I will put the amp back together, and play it a few more times, and think about what to do.

For the time being perhaps we should close this case - and to quote Spinal Tap - 'best left unsolved'.

I plan to return to this forum soon with my next project - pimping up a Roland Micro Cube.

Thanks
Jim

phatt

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Re: Noob with a soldering iron
« Reply #81 on: February 11, 2015, 08:27:17 AM »
Hi Jim,
        I'd be adding that ~100p cap across the mixer FB before you give up, it will kill off a lot of hiss.
I've done this many times as a lot of high freq is not needed for good bluesy sounds.

caps were missing anyway which maybe all that was wrong,,,so worth a try. :tu:

The success of Amps like early Marshall's is due to not having a lot of top end.
Players like Santana have only just enough bandwidth to cover the fundamentals and maybe one extra octave. If you add all the extra freq it just destroys the tone and the magic is lost. :'(

If I recall you wanted a more blues tone anyway so it might actually improve the sound for you.
just a thought, Phil.
« Last Edit: February 11, 2015, 08:29:59 AM by phatt »

g1

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Re: Noob with a soldering iron
« Reply #82 on: February 11, 2015, 12:05:55 PM »
  Also, have you tried contacting the designer and asking him about it?  He sounds like a pretty helpful individual and may have some suggestions.

scmitche

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Re: Noob with a soldering iron
« Reply #83 on: January 21, 2016, 09:12:12 PM »
Thanks guys
I will have a think what to do with this amp. I have learnt an enormous amount from working on it, but perhaps it's just a noisy amp. I have other SS amps around, and I'm telling you this one has an annoying hiss. It has been a 'project' - with the fantastic help of you guys - to see if we could quieten this amp. I guess if I got it to a technician maybe the source of the hiss could be found, maybe he'd just say it's a hissy amp.

Criticisms of my recording - unfortunately I could only use my phone, so that's not a great start. Yes I put the phone near the amp, but there's already noise and hiss from the phone, so there was no point in having the phone at a distance where you couldn't separate it's own hiss from the hiss of the amp. Believe me from one metre the hiss on this amp is noticeable.

I will put the amp back together, and play it a few more times, and think about what to do.

For the time being perhaps we should close this case - and to quote Spinal Tap - 'best left unsolved'.

I plan to return to this forum soon with my next project - pimping up a Roland Micro Cube.

Thanks
Jim


I wish I'd seen this last year as I could have been helping you.
Considering IC1a and IC1b in your posted circuit diagram you can change the 1Meg feedback resistors to 100k,  the 100k resistors to 10k, increase your added 22pF to 220pF and increase the 10nF not to 100nF as you would expect  but to 47nF for a better sound.
All these ideas can be seen in the later model Award-Session HiFlex140 preamp circuit and found in the S100R112 circuit diagram on the Award-Session site. My own SG30 is now very quiet.

Regards,
Steve