Please help me identify this booster and reduce the hiss

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Please help me identify this booster and reduce the hiss

Postby Cub » 06 Dec 2013, 13:33

Blimey ! Perform a couple of mods of your mates' toob-reamers, build a Muff or two, show up with a bare aluminium box at jam night and everybody thinks you're some kind of expert ! :lol:

So my mate dropped off this nameless clean boost yesterday with the request if I could "make it quieter." I agree, there's an unreasonable amount of hiss coming from this thing, more than most dirt boxes. Also, there's not a whole lot of boost on tap, even with the level at 10, but that's not much of a problem. He uses it more as a tone-sweetener for his single coil guitars instead of a volume boost.

Does any of you recognise this booster or perhaps a classic that may have inspired it ?
What would help bring the hiss down ?

thank you !
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Re: Please help me identify this booster and reduce the hiss

Postby ggedamed » 06 Dec 2013, 22:46

This is what I could make from your pictures:
fsb_cub_booster.jpg

fsb_cub_booster.png

It looks to me like an early design of Jack Deville. Not the best one. The input cap and almost all resistors are way too large and the second op amp has inputs tied together and no feedback loop.
If I were you I would:
- lower R1 and R2 10 to 30 times (and the associated potentiometer)
- lower C3 to 22n..100n
- disconnect pin5 from GND and connect it to VR, and disconnect pin6 from GND and connect it to pin7; the second op amp output would then be available as a much better VR
- lower R4 to 100R..220R

Or you might look at these:
MXR - Microamp (you wanted a classic)
Mr. Black Boost Tiger (after he learned a few things in the topic above)
Active pickup (differential)
Another active pickup (differential too)
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Cub (09 Dec 2013, 11:02)
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Re: Please help me identify this booster and reduce the hiss

Postby Jack Deville » 07 Dec 2013, 10:42

Not my design.

Please keep your ignorant commentary to yourself.
I'm a "professional."
Buy my products and make me rich.

www.jackdeville.com
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Re: Please help me identify this booster and reduce the hiss

Postby ggedamed » 07 Dec 2013, 16:22

I didn't say it's yours. I hope you'll forgive me for being deceived by a few similarities with this one. You don't see everyday op amp boosters with the gain pot between the inverting input and the ground, and with the second op amp with both inputs tied together.
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Re: Please help me identify this booster and reduce the hiss

Postby Cub » 09 Dec 2013, 11:02

Thank you very much, ggedamed. Your replies are thorough, friendly and very helpful. :applause:

As weirs as the values for the components may seem, it does sound really nice like this. It's just the hiss that needs to be tamed.
Will your suggested mods alter the tone or just take care of the hiss ?
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Re: Please help me identify this booster and reduce the hiss

Postby ggedamed » 10 Dec 2013, 01:39

Hmm, I just noticed the image with values. So the actual circuit would be something like this:
fsb_cub_booster.png


This is what I would do:
fsb_cub_booster_mod.png

As you can see, I reduced some resistor values (this should have an effect on hiss), the input capacitor and the gain pot capacitor (one should try to limit the available bandwith to what the circuit actually uses). I've increased the output load resistor because it will be anyway in paralell with the input resistance of the following stage and the output load of an op amp should not go under 10k, just to be on the safe side. You could try swaping the IC with a low noise one (I used an OPA2277, becase has fairly low noise and a low suply current and, of course, because I know I have them in my drawer).

This is the AC analysis of the mods compared to the original:
fsb_cub_booster_ac-analysis.png

fsb_cub_booster_mod_ac-analysis.png


What puzzles me is that you should not have too much hiss in this circuit. It doesn't have significant gain AND the circuit itself looks like a lowpass filter starting to atenuate from around 1kHz. Are you sure the hiss is from the booster? Maybe the noise is produced before or after the booster.
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Re: Please help me identify this booster and reduce the hiss

Postby Cub » 10 Dec 2013, 18:39

Wow, thanks again for going to such great lengths to help me out ! There really is an incredible amount of hiss, too much for it to be useful with anything other than a clean sound, if you ask me. This is with a fresh battery and without any other effects.

I asked my mate about it and this was his reply:
"It's been like this since the moment I got it: a lot of hiss, louder than unity at 0 and not much louder than that at 10. Not loud enough to make a clean amp go crunchy or to boost a crunchy amp for leads. But the volume is not an issue, I only use it to sweeten up the sound. It's far from transparent and colours the sound in a brilliant way, especially my single coil guitars."

Could it be a faulty component then ? I'll take out the DMM tomorrow and take measurements all across the board. That might tell us something.
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Re: Please help me identify this booster and reduce the hiss

Postby Cub » 11 Dec 2013, 15:02

There, I believe I got all of them.
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Re: Please help me identify this booster and reduce the hiss

Postby ggedamed » 12 Dec 2013, 00:51

Hmm, some weird voltages there.
First, what does it mean "when power is applied"? When you stomp on the switch or when you connect the PSU?
The voltages on the both sides of C3 are weird: the one on the T pin depends on what is connected to it, so I'll not comment that.
But the one on pin 3 of the IC should be Vr, by R3. The source of the offset can be either the IC (which means the IC is defective) - try replacing it with a good socket -, or C3 leaking some voltage through it (if such voltage exists on the T pin of the input jack) - try replacing it with a non-polarized capacitor, any value down to 22n will work.
The other weird area is on the GAIN pot. This one is a real mistery: on the IC side it has 4.59V and on Vr side it has 4.54V. So it gets some voltage from somewhere else. I would start searching from the pin 1 of the pot - first check that is no contact (measure the resistance) between pin 1 of the pot and ground (not Vr).

Actually, for starters I would desolder C3 and the GAIN pot wires and see what happens to the voltages. If the voltage on pin 3 of the IC is still far away from pin 2 I would desolder the IC and replace it with a good socket.

Also, maybe I was not clear: except the high frequencies filtering and the hiss (how can you have both is beyond me) there's no mojo in the circuit.

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Re: Please help me identify this booster and reduce the hiss

Postby Cub » 12 Dec 2013, 20:26

I took the measurements with a fresh battery and when I went back to check on some the findings, some measured were lower already. The IC pins are the first thing I measured, then the rest. Perhaps that causes the 0,05v difference ?
All measurements were taken With the a dummy mono plug in the input and nothing else connected, the negative side of C3 started at about -2,00 when I inserted the plug.

After some digging, I found my Dunlop ECB003 wall-wart and took some more consistant measurements today. I connected the pedal to a guitar and a running amp as well this time to see if made a difference. It sure did ! The weirdness at C3 is gone, there's no voltage on any pins of the input and output jacks. But now, it's weird between pin 2 and 3, I suspect the anti parallel diodes cause this ?

There's no OPA2277 in my parts drawer at the moment, but I did a little shoot-out with the chips I do have. A different TI RC4558P had the same amount of hiss, as did a JRC4558D, an RCA1458N and an ST LM358AN. An ST LM833N possibly had even more hiss.
There was slightly less hiss with a TI LF353P, a BB OPA2134PA and a BB OPA2604AP, but I had to swap them and a TI RC4558P back and forth a number of times to be certain, the difference was really that slight.

I'm starting to feel I'm a bit out of my league, hacking up the board for the opamp mods. Perhaps it's better to get a nice MXR Micro Amp board from GGG, use a single opamp, tie some diodes to the inverting and non inverting inputs and fill the rest of the board with your suggested values.
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Re: Please help me identify this booster and reduce the hiss

Postby ggedamed » 13 Dec 2013, 23:54

Man, this makes no sense :scratch:. What's your measuring tool? An usual DMM (digital multimeter)? At least you found that the IC is not the hiss source.
BTW, I still don't understand what means "when power is applied" in "Falls from 0.03 to 0.00 when power is applied" on the schematic.

At this point I would say a new circuit built from scratch is the fastest option. But the failure would haunt me forever :lol:.

If you still want to put up with this PCB, you could (I would) disconnect C3 and the GAIN pot and take out the IC and measure again.

FYI, there are dual op amp PCBs on tonepad.com and AMZFX (this one allows you more circuits on it).
It seems the tonepad.com pdf cannot be downloaded, so here it is:
tonepad_microampdistplus.pdf

You can find some information about MXR Microamp on electrosmash.com.

The diodes are there only for protection, they can be removed with no effect on tone.

EDIT: if you have it, TLC2262 is a good IC to try - lower noise than TL072, eats only 0.5mA and is rail-to-rail. Just remember it cannot be fed more than 16V.
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Re: Please help me identify this booster and reduce the hiss

Postby Cub » 15 Dec 2013, 18:35

Blimey, swapping parts in a pedal for a different tone is one thing, building a new one is another, but troubleshooting somebody else's handywork ? Apparently, that's too much for a n00b like me. :slap: Despite your great help, thank you for staying so friendly, helpful and patient with me

When I took the measurements with a battery, I put the DMM on the negative side of C3, then inserted the dummy plug in the input (applying power to the circuit) and watched the voltage rise from about -2,00v to to 0,00v.

Then when I had a guitar and a running amp connected to it, I put the DMM on the negative side of C3 again, inserted the jack from the Dunlop power supply (applying power) and saw the voltage drop from 0,03v to 0,00v.

My mate had a gig this weekend, so I gave the pedal back with the promise that I'd build him a better one in the new year. I ordered some boards from TonePad and will keep you posted.

Mulţumesc mult ! :applause:
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