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stahlhart
Platinum Member
1318 Posts |
 Posted - 11/15/2005 : 19:26:15
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This one's got me a little baffled. I have a MIJ DS-1, and in an effort to convert it to a cleaner boost pedal (I'd prefer to get overdrive from my tube amp), I removed D4 and D5.
The pedal continues to distort pretty much as it did with the diodes in...
Is this mod more for the Taiwanese DS-1 circuit than the Japanese one?
C.K.
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Astatine
Copper Member
13 Posts |
Posted - 11/17/2005 : 12:42:59
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| If I were you, id keep the valuable MIJ DS-1 in its original state and just buy a SD-1. They're really great clean boosts, and they only cost around $40. You could always sell your DS-1 if you dont use it, im sure you would be able to raise the cash for an SD-1 and some more besides! |
Edited by - Astatine on 11/17/2005 12:43:37 |
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stinkfoot
Silver Member
Sweden
181 Posts |
Posted - 11/18/2005 : 23:29:25
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With D4 and D5 taken out (not bridged or jumpered, just plain removed), you have eliminated all distortion and compression the diodes used to generate. If you keep the gain/dist knob high enough, the op-amp will distort (as you noticed). The trick to getting the clean boost you're after is to use the extra output level the pedal gained when you removed the diodes - keep the gain/dist knob low (9 o'clock or below) and crank the output volume knob instead. That should give you plenty of clean boost.
/Andreas |
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stahlhart
Platinum Member
1318 Posts |
Posted - 11/19/2005 : 21:27:46
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Andreas:
Okay; thanks much... it just kind of surprised me, as I thought that the bulk of the clipping in the pedal was on account of those diodes -- didn't realize that it was also happening elsewhere in the circuit.
Mabye the clipping is more dependent on D4 and D5 in the simpler Taiwanese circuit...?
Astatine:
Yes, I may go that route, or possibly the MXR Micro Amp (I used to have one of these many years ago). I just happened to have the DS-1 here and wasn't really doing much with it, and thought I'd experiment a little. I can restore it back to its original state -- you're right, I should never consider permanent modifications for a MIJ pedal...
I'm much more of a fan of clipping from a tube amp than a pedal. I've got a somewhat rare Ampeg GU-12 that is *really* clean on its own, and am looking at a combination of gain boost tweaks in its circuitry, along with feeding it a hefty input signal, to warm up the tone a bit.
C.K.
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stinkfoot
Silver Member
Sweden
181 Posts |
Posted - 11/20/2005 : 00:30:40
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The clipping is dependent on the diodes as long as they are there - as they clip the signal, they also limit the signal's max swing, largely keeping the op-amp itself from distorting. When you remove the diodes, there's nothing stopping the op-amp from going into clipping, so you can't expect it (either the new or old one) to be clean with the gain fully up.
But since you've removed the clipping/limiting diodes, you don't have to use as much gain in the op-amp to get the output level you want. So you turn the gain down and the output up, and you've got yourself a clean boost. Mind you, an MXR Micro Amp will clip too, if you push it too hard. After all, it is basically a Distortion+ with the clipping diodes and output level control removed. The fact that it has no output/master control means that you normally will never crank the op-amp into clipping, since the output level at that point would be far too big. But if you were to max the gain knob on the Micro Amp (and preferably add an output level control, to keep the signal level reasonable), you'd hear it distort quite a bit.
So, to make your diode-less DS-1 work like a Micro Amp, all you have to do is crank the output level knob and control the boost from the gain/distortion knob.
/Andreas |
Edited by - stinkfoot on 11/20/2005 00:31:41 |
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bossarea
Forum Admin
United Kingdom
3652 Posts |
Posted - 11/20/2005 : 18:19:19
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I had a look at this found that this opamp should never distort as its amplification should never go above 1. Now I'm known to get these things wrong so there's probably more than 50% chance that I've done it again.
I looked at the schematic and simplified the circuit to the following
After some math I came up with the following expression for the amplification
Av = Uo/Ui = (R2 + R3) / (R1 + R3)
R1 is 100kOhm
R2 is a part of VR1
R3 is the other part of VR1 + the 4.7kOhm resistor in series with it.
Let's look at the min and max situation of the dist pot.
1) VR1 completely counter clockwise (Dist = 0)
R1 = 100k, R2 = 0, R3 = 100k + 4.7k
Av ~ 0.5
2) VR1 completely clockwise (Dist = 10)
R1 = 100k, R2 = 100k, R3 = 4.7k
Av = 1
If this is correct then the diodes should provide all distortion as they clip the signal when Av approaches 1 but not when Av is closer to 0.5.
Did I make to many simplifications? Are my calculations wrong? Maybe a bit of both?  |
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stinkfoot
Silver Member
Sweden
181 Posts |
Posted - 11/20/2005 : 22:53:39
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That kind of stuff is way over my head, but... I do think the op-amp (or op-amps, as in the early MIJ version) would have to be able to go above unity gain. If not, there would be precious little output signal left after the diode clipping stage, seeing as the DS-1 has no volume recovery stage. The output level control is just a passive voltage divider, with no other amplification stages after it (just the 1:1 output buffer).
/Andreas |
Edited by - stinkfoot on 11/20/2005 22:55:34 |
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bossarea
Forum Admin
United Kingdom
3652 Posts |
Posted - 11/20/2005 : 23:32:22
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The output voltage from the opamp shouldn't be more than 0.5 - 1V. Anything more than that and D4 and D5 will always clip. Besides that they smoke with a forward voltage of 1.2V. Q2 can potentially drive the voltage up to that level which would negate the need for the opamp to have more than unity gain.
However, Stahlhart has already tested and found that the circuit distorts without the diodes and there aren't anything after the opamp that can generate distortion.
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stahlhart
Platinum Member
1318 Posts |
Posted - 11/21/2005 : 03:14:19
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The one bench tool I'm sorely lacking at the moment is an oscilloscope, but I plan to get one between now and next summer sometime... it might be interesting to look at waveforms through the circuit to see where the clipping actually occurs.
If no one else in the forum ever goes this route (and if anyone still remembers this discussion down the road), I'll report back once I've had the chance to revisit this properly on a workbench...
Thanks very much to everyone for the feedback!
C.K.
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TrevDinah
Copper Member
USA
3 Posts |
Posted - 03/06/2006 : 23:56:53
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quote:
Originally posted by stahlhart
This one's got me a little baffled. I have a MIJ DS-1, and in an effort to convert it to a cleaner boost pedal (I'd prefer to get overdrive from my tube amp), I removed D4 and D5.
The pedal continues to distort pretty much as it did with the diodes in...
Is this mod more for the Taiwanese DS-1 circuit than the Japanese one?
C.K.
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eeone
Bronze Member
Yugoslavia
121 Posts |
Posted - 03/07/2006 : 01:10:57
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About the op-amps.
First, as do all semi-conductor components, op-amps have enormously big tolerance. And here's the thing - gain of the most of the op-amps today vary between 1e6 and 2e6, sometimes even more.
Now, the op-amps have two power supply connections, Vcc (usually positive) and Vee (usually negative), (for example, if op-amp is made using bipolar transistors). The output voltage of an op-amp cannot exceed Vcc nor fall below Vee. If it does, it is said that the amplifier is in saturation and the output voltage is either Vcc or Vee. So, if you put Vee=0 op-amp will clip everything below 0v on its input. (for example a sine wave would have only positive half-periods).
Considering these two facts, it is logical that even the smallest change in input voltage would cause an op-amp to saturate, thus making it absolutely unusable for any practical use.
And then some smart people invented feedback. One part of the signal is being returned to the input of an op-amp, as shown on the picture of one of bossarea's previous posts (in which the Av = (R2 + R3)/R3 ) . There are two types of feedback - positive and negative.
The positive feedback is rarely used. The signal is returned to the positive input of the op-amp. In most cases because it increases already huge gain and secondly because these circuits tend to oscillate a lot, which you don't want when you make an amplification circuit, like distortion. (And this is why I decided to post all this since the circuit on the picture uses positive feedback, which I found was quite odd).
The negative feedback is the one (signal returned to negative input). Overall gain (Vo/Vi) is easilly adjusted, using only resistors. Also, you can make simple filters if you use capacitors.
As for the ds-1 mod I'm quite sure that the op-amp itself clips the signal. Before the diodes were removed they where clipping the signal at about 0.6 - 0.7 V or even 1.2V as bossarea mentioned, which is less than what it takes for an op-amp to saturate.
There, hope this helps. (I'll check all this stuff tomorrow.) |
Edited by - eeone on 03/07/2006 01:14:01 |
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DS-1 Clean Boost Mod(s)?
