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copper
Bronze Member
USA
73 Posts |
 Posted - 03/23/2009 : 02:21:38
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| If I want more gian from my SD-1, what should I tweak? I think I read somewhere that R6 affects this, but looking at the schematic it looks like I should be checking out R5. Hmmmmm |
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hot tubes
Silver Member
Canada
162 Posts |
Posted - 03/23/2009 : 04:10:50
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i put a switch here and put the 4.7 k on one side then a 2.2 k.
4.7 factory value
2.2 high gain value |
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leonard d rock
Silver Member
Philippines
301 Posts |
Posted - 03/23/2009 : 06:07:49
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hello, can you elaborate further on this, perhaps a diagram too. thanks!
ldr |
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cctsim
Silver Member
United Kingdom
418 Posts |
Posted - 03/23/2009 : 15:59:27
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The gain for SD-1 is controlled by a voltage amplifier (see fig. below). The gain for this amplifier is given by the following equation:
H(j*w)=1+(R5 + VR1)/(R6 + 1/(j*w*C3))
where w=2*pi*f and VR1 is the pot value for the drive control.
To increase the gain you can reduce the value of R6 (denominator becomes smaller -> ratio increases -> gain increases).
One thing to remember is that if you want to keep the frequency response similar to the original, you need to adjust C3 to have the same frequency cut-off:
fc = 1/(2*pi*R6*C3)
In other words, if you change the R6 to 2.2k then to keep the same cut off you need to change C3. The original frequency cut off is:
fc_orig = 1/(2*pi*R6*C3) = 1/(2*pi*4.7e3*0.047e-6) = 720.48 Hz
Hence the new value of C3 is
C3_new = 1 / (2*pi*R6_new*fc_orig)= 1/(2*pi*2.2e3*720.48) = 1.0041e-07 ~ 0.1 uF
which is approximately 0.1 uF.
Here is the original frequency response C3=0.047uF and R6=4.7k:
Here is the modified frequency response C3=0.1uF and R6=2.2k:
Best Regards
cctsim
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Edited by - cctsim on 03/23/2009 23:09:40 |
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nosi0
Gold Member
Netherlands
511 Posts |
Posted - 03/23/2009 : 16:18:45
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Wow very nice explanation!
Nosi |
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hot tubes
Silver Member
Canada
162 Posts |
Posted - 03/23/2009 : 16:23:44
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| i did not know that also , thanks !! |
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Laurie
Double Platinum Member
Canada
4854 Posts |
Posted - 03/23/2009 : 20:47:01
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| Wow cctsim! You rock!! THank you for that! |
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cctsim
Silver Member
United Kingdom
418 Posts |
Posted - 03/23/2009 : 23:09:16
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I am glad you liked the analysis Guys, thanks.
Theoretically, you can increase the gain by increasing R5 too. However, I wouldn't recommend to do that as the VR1 pot is already 1Meg and large resistor values result in enhanced noise.
The Keely 5* GE mode changes R6 to 2.4k and C3.to 0.1uF. The cut off is ~663 Hz claiming to remove the "mid hump" a bit.
Personally, I like the characteristics of SD-1 and try to keep the original frequency response. |
Edited by - cctsim on 03/24/2009 04:50:10 |
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leonard d rock
Silver Member
Philippines
301 Posts |
Posted - 03/24/2009 : 07:40:15
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| thanks for your explanation! |
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copper
Bronze Member
USA
73 Posts |
Posted - 03/25/2009 : 01:12:06
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Thanks guys. 
Cheers..... |
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ChristoMephisto
Platinum Member
Canada
1288 Posts |
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hot tubes
Silver Member
Canada
162 Posts |
Posted - 03/27/2009 : 03:43:41
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| can the above gain mod be done to the DS1 also ? |
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cctsim
Silver Member
United Kingdom
418 Posts |
Posted - 03/27/2009 : 15:47:44
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Yes, the transfer function for DS1 opamp stage is very similar, however, there is one extra capacitor C7:
There is 1 highpass cut-off frequency (nominator):
and 2 lowpass cut-off frequencies (denominator):
They all depend on the rotation of the DIST knob, e.g. k takes values between 0 and 1, with 0 -> 0% and 1 -> 100%.
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Dr. Bob
Moderator
Australia
6593 Posts |
Posted - 03/27/2009 : 15:55:35
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GOD said.
quote:
Originally posted by cctsim
Yes, the transfer function for DS1 opamp stage is very similar, however, there is one extra capacitor C7:
There is 1 highpass cut-off frequency (nominator):
and 2 lowpass cut-off frequencies (denominator):
They all depend on the rotation of the DIST knob, e.g. k takes values between 0 and 1, with 0 -> 0% and 1 -> 100%.
And Then There Was LIGHT ...   
Regards Dr. Bob 
4999 |
Edited by - Dr. Bob on 03/27/2009 15:57:35 |
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cctsim
Silver Member
United Kingdom
418 Posts |
Posted - 03/27/2009 : 16:53:00
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Sorry I forgot to turn the switch on :
Here is the Keeley SEM/ULTRA C7=220p, C8=1uF, R13=2.4k
I am working on a DSP implementation of various boss overdrives and got carried away. No more math litter, I promise. |
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MoonWatcher
Bronze Member
USA
98 Posts |
Posted - 03/29/2009 : 18:21:10
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I've actually never been a big fan of increasing gain at that point, since it typically extends the op amp into a range where you're primarily getting unusable distortion, and it also drives up the amount of noise, as well.
I'll occasionally go to 3.3K for R6, but actually prefer to go higher, either to 6.8K or 10K, to reduce extra noise. And I do adjust C3 appreciably, by my tastes are different regarding the high pass shelf. I also think that different amounts of median drive require a different cap value for C3. If you typically use less drive, a bigger cap works well. As the drive goes up, you need to cut more bass to prevent intermodulation (unless you like that sound).
I typically reduce R7, R10, and R11. If you reduce R7, you obviously have to play with the value of C4, and occasionally C5 as well.
You can also exploit extra gain by increasing R9, but since it's tied to the tone circuit, you ultimately can end up with a super treble booster, since C5 and VR3 are synonymous to C3 and R6.
Yet another trick to increase gain is to increase the clipping threshold at D4, D5, and D6. Probably the easiest way to do it would be LED's for D4 and D5. But this usually increases the threshold too much, so the next best thing is to run more 1N4148's in series. An even more incremental approach would be to run 1N34A's or BAT41's in series. But note that by increasing the threshold that you reduce compression. You also lose some of the clamp effect that the diodes provide, so max settings will probably sound less than musical. |
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SD-1 more gain mod
