what part of this schedule is bypass related and can be removed? - diyAudio
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Old 22nd August 2011, 10:25 AM   #1
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Default what part of this schedule is bypass related and can be removed?

Hi all

iam working on a 19" distortion project, and I would like to use this schedule I traced from my Rocktron Austin Gold overdrive. Could you guys help me to figure out wich part is the bypass and can be left out?

Tnx a lot

Erik
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Old 22nd August 2011, 12:27 PM   #2
godfrey is offline godfrey  South Africa
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Hi Erik

To start with, you can leave out all the stuff circled in red, and add in the connection shown in blue. Then you could probably simplify it a bit more by leaving out C13, R9 and R7.

I think you missed an earth connection. I've added it in, in the green circle.

Cheers - Godfrey
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Old 22nd August 2011, 08:44 PM   #3
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Thanks a lot!

I have removed hen marked selections and added the ground. I got a few questions:

1. Isn't the blue line that you draw obsolete now?
2. I marked two more parts of IC 3 & 4, can't they go as well?

Cheers

Erik
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Last edited by erikb1971; 22nd August 2011 at 08:52 PM.
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Old 23rd August 2011, 02:25 PM   #4
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by erikb1971 View Post
I have removed hen marked selections and added the ground.
Sorry, I made a mistake. That should connect to VCC/2, not ground.
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Originally Posted by erikb1971 View Post
1. Isn't the blue line that you draw obsolete now?
Yes, and where you deleted part of the circuit below that, it should be left open, not connected from one side to the other.
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Originally Posted by erikb1971 View Post
2. I marked two more parts of IC 3 & 4, can't they go as well?
Yes, you don't need IC 3 and 4 at all, they were just to switch between bypass and overdrive.

You can leave out C13, R9 and R7 as well.
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Old 23rd August 2011, 02:44 PM   #5
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Wow.. thank you very much!

I have another question for you, if you do not mind. In the schematic there is a pre-bass pot. Could you from the schematic see what that does exactly?
The reason Iam asking is the following: the pre bass seems to add some low frequencies before distortion. I would like to be able to mod it so it can boosts higher frequencies as well...

Thanks a lot again!

Erik
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Old 25th August 2011, 08:59 AM   #6
godfrey is offline godfrey  South Africa
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OK, that's a tough one. I can see what it does electrically, but it's hard to imagine what it sounds like.

The Prebass potentiometer, together with R3 and C5 roll off the bass before the signal gets distorted. If you wish you could turn that control further one way or the other, you can get that effect by increasing or decreasing the value of C5.

The other part of the circuit that behaves differently at different frequencies is the combination of R4 and C8 (right at the top of the schematic). As an experiment, you could try changing the value if that capacitor and see what happens to the sound.

In either case, a slight change in value won't do much. I'd try at least doubling and halving them.

edit: ...or as a totally random (but zero-cost) experiment, just swap those two capacitors and see what happens.

Last edited by godfrey; 25th August 2011 at 09:03 AM.
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Old 25th August 2011, 10:37 AM   #7
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OK, that's a tough one. I can see what it does electrically, but it's hard to imagine what it sounds like.
Good thing I have one of these pedals right here then. together we sould be able to figure it out. And we have help from the rocktron website:

"Unique to the Austin Gold is its "Pre-Bass" control. Other typical overdrive pedals have a fixed value which cannot be altered. But the Austin's Pre-Bass control allows you to cut the pre-distortion low frequency information, eliminating problems with too much "fuzzy" bass. This control also allows you to increase the pre-distortion frequencies when the tone is too weak or thin."

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The Prebass potentiometer, together with R3 and C5 roll off the bass before the signal gets distorted. If you wish you could turn that control further one way or the other, you can get that effect by increasing or decreasing the value of C5.
That seems to be right with what rocktron says and with what I hear. With the pre bass fully clockwise, it has almost the same amount of bass as the original. Moving it counter clockwise reduces the amount of bass. Funny thing is that when I put level and pre bass all the way up and drive all the way down it is almost identically in sound, but even with that setting, the austin gold seems to cut some sub low freq... Tomorrow Iam going to make a sound clip with a freq analysis to take a closer look at that!

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The other part of the circuit that behaves differently at different frequencies is the combination of R4 and C8 (right at the top of the schematic).
So that part reacts differently when you turn the pre bass pot... I wonder what that could be since I do not seem to hear anything but the bass cut, ....

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Originally Posted by godfrey View Post
As an experiment, you could try changing the value if that capacitor and see what happens to the sound.

In either case, a slight change in value won't do much. I'd try at least doubling and halving them.

edit: ...or as a totally random (but zero-cost) experiment, just swap those two capacitors and see what happens.
I love totally random but zero costs experiments! I will get to that tonight or tomorrow....

One small other question: what does D1 do? It seems to be broken on my austin gold but it works fine!

Last edited by erikb1971; 25th August 2011 at 10:41 AM.
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Old 25th August 2011, 03:38 PM   #8
godfrey is offline godfrey  South Africa
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So that part reacts differently when you turn the pre bass pot... I wonder what that could be since I do not seem to hear anything but the bass cut, ....
No, R4 and C8 act independently of the pre-bass pot, but they also make the circuit behave differently at different frequencies.
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One small other question: what does D1 do? It seems to be broken on my austin gold but it works fine!
D1 is just to protect the circuit if the battery or DC power supply is connected backwards. Not essential, but probably worth replacing if you get a chance. It's a bit of a crude arrangement, designed to kill your battery before it can kill the electronics, but there you are.

Edit:
Quote:
Originally Posted by erikb1971 View Post
Funny thing is that when I put level and pre bass all the way up and drive all the way down it is almost identically in sound, but even with that setting, the austin gold seems to cut some sub low freq...
Aha! That's probably R4 and C8 doing their thing. Well, partly. The pre-bass control always cuts the low freq's a bit, but C8 cuts them a lot. Try replacing C8 with a piece of wire to see the effect it has. That'll screw up the DC offset on the opamp a bit too, but hopefully not too badly.

Last edited by godfrey; 25th August 2011 at 03:56 PM.
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Old 25th August 2011, 07:09 PM   #9
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I replaced C8 with a wire... it had no effect of the bass cut, but the drive pot now has no effect anymore what so ever
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Old 26th August 2011, 08:32 PM   #10
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fixed the problem with c8, and I have replaced it with one half and with one double the value, and it does not have any effect onb the bass cut at all....
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