Hi,
I'm looking for a source of affordable 2% tolerance 680nF caps.
they're to fit in Keith Snook's Quad 405 DCD Mod4 design, so should have a relatively small dot pitch of around 1cm (not absolutely critical).
They should also be available in small quantities (I need 4) at an affordable price.
Preferred materials, (matallized?)polypropylene/polyester film.
Any info gladly appreciated
I'm looking for a source of affordable 2% tolerance 680nF caps.
they're to fit in Keith Snook's Quad 405 DCD Mod4 design, so should have a relatively small dot pitch of around 1cm (not absolutely critical).
They should also be available in small quantities (I need 4) at an affordable price.
Preferred materials, (matallized?)polypropylene/polyester film.
Any info gladly appreciated
Finding capacitors with factory tolerances tighter than 5% isn't easy. Of course you can always measure several and use the ones that come closest to the ideal value. I don't know the application - is the absolute tolerance critical, or is it more important that two (or more) capacitors be closely matched?
In the U.S., Mouser has polypropylene through-hole parts in 2% tolerance and 15mm lead spacing from Vishay and Kemet. DigiKey has Panasonic polypropylenes with 3% tolerance and 20 mm lead spacing. Both vendors carry parts with 5% tolerance from several vendors and lead spacing from 5mm to 25 mm.
Dale
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The Quad Mod4 schematic linked here shows the cap, C4. Only one is required per channel as it's the compensating cap. for
the feedforward bridge inductance and hence affects linearity and gain. 2% may not be an unreasonable spec. assuming L2 is
somewhere near that tolerance region too.
the feedforward bridge inductance and hence affects linearity and gain. 2% may not be an unreasonable spec. assuming L2 is
somewhere near that tolerance region too.
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Thanks all for the links.
L2 is tricky too. Inductors have as a rule 20% factory tolerance, but I bought mine through Keith Snook whi has removed one or two windings to come close to the 2.8 mH spec. (I don't know if he arrived at the number of windings through theory or measurement though.
The C1 and C4 caps he has supplied are pretty basic 10% components although he says himself that they should ideally be closer, and has specified 2% on the schematic. 5% looks pretty easy to source. Thanks everyone for the discussion and links.
L2 is tricky too. Inductors have as a rule 20% factory tolerance, but I bought mine through Keith Snook whi has removed one or two windings to come close to the 2.8 mH spec. (I don't know if he arrived at the number of windings through theory or measurement though.
The C1 and C4 caps he has supplied are pretty basic 10% components although he says himself that they should ideally be closer, and has specified 2% on the schematic. 5% looks pretty easy to source. Thanks everyone for the discussion and links.
I hate to say it, but if the stability is based on 2% parts, it is a poorly designed circuit. Now, he may have said 2% because in the real world, when you add in voltage bias, humidity, and temp, it will probably remain within5%. for a film. Tolerance on caps is not like on a resistor. The capacitance of all the transistors will drift more than that with voltage and temp.
I suggest you do a SPICE simulation and see where you stand. I would also pay attention to the warnings in the upper left corner. That is a hint. It is tempting to minimize the compensation to squeeze every last bit of distortion down, but not good in the real world.
I suggest you do a SPICE simulation and see where you stand. I would also pay attention to the warnings in the upper left corner. That is a hint. It is tempting to minimize the compensation to squeeze every last bit of distortion down, but not good in the real world.
I don't think this is to do with stability so much as Low pass filter and frequency response of the input stage.
Keith Snook:
C1 (680nF) and R3 (22kΩ) form a high pass filter at about 10.6Hz (a time constant of 15ms) which prevents d.c. on the input upsetting the amplifier ~ R6 ~ R3 and R4 set the stage gain at 15x but when C4 (47nF) is back in series with R6 (330kΩ) the frequency response of the input stage becomes in theory flat down to a few mHz because the C4+R6 time constant is also 15ms and negates the effect of C1+R3 for as low as the amplifier gain holds out
In Mod 4 both C1 and C4 are set to 680nF. I believe the most important aspect is that both should be of the same type.
Keith Snook:
C1 (680nF) and R3 (22kΩ) form a high pass filter at about 10.6Hz (a time constant of 15ms) which prevents d.c. on the input upsetting the amplifier ~ R6 ~ R3 and R4 set the stage gain at 15x but when C4 (47nF) is back in series with R6 (330kΩ) the frequency response of the input stage becomes in theory flat down to a few mHz because the C4+R6 time constant is also 15ms and negates the effect of C1+R3 for as low as the amplifier gain holds out
In Mod 4 both C1 and C4 are set to 680nF. I believe the most important aspect is that both should be of the same type.
He already found them: BFC241676804 Vishay / BC Components | Mouser and F461BY684G400L Kemet | Mouser
The trouble is probably you were searching for pF only. Caps are listed uF as well. You have to do the metric conversion. Hope that helps...
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If I understand keith Snook's comment correctly the values affect the 15ms time constant of C1+R3 and C4+R6 (which should match). Unfortunately I'm no theoretician so can't say how upsetting one side of this by, say 20% on either cap - or both, in opposite directions - would affect that time constant and thus the frequency response of the amplifier.
the input coupling RC is coordinated with the feedback RC - I assume the intent is for pole-zero cancellation
so ideally the respective RC time constants should match to avoid a bump or dip in the frequency response
practically there is little point in extreme accuracy at 10 Hz - several dB, 10-20 % mismatch won't reach audible thresholds that low
we just don't hear, resolve small frequency response diffierences that low - even if they are easily measured
Clark, David L., "High-Resolution Subjective Testing Using a Double-Blind Comparator", Journal of the Audio Engineering Society, Vol. 30 No. 5, May 1982, pp. 330-338
ABX Amplitude vs. Frequency Matching Criteria
additionally room modes, speaker roll off, location, type make for 10s of dB roughness in response in the modal region
so ideally the respective RC time constants should match to avoid a bump or dip in the frequency response
practically there is little point in extreme accuracy at 10 Hz - several dB, 10-20 % mismatch won't reach audible thresholds that low
we just don't hear, resolve small frequency response diffierences that low - even if they are easily measured
Clark, David L., "High-Resolution Subjective Testing Using a Double-Blind Comparator", Journal of the Audio Engineering Society, Vol. 30 No. 5, May 1982, pp. 330-338
ABX Amplitude vs. Frequency Matching Criteria
additionally room modes, speaker roll off, location, type make for 10s of dB roughness in response in the modal region
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Hi again,
I was searching for 0.68uF but there seems to be a problem finding anything with a dot pitch of less than 15mm, although I have found some 5% components that are okay.
Here's the relevant section opf the pcb, showing mounting holes for C1: 5 / 7.5 / 10mm
and C4: 5 / 7.5mm:
My best/only option for mounting 15mm dot pitch components would be to mount them on reverse side, but I guess that would be okay.
I was searching for 0.68uF but there seems to be a problem finding anything with a dot pitch of less than 15mm, although I have found some 5% components that are okay.
Here's the relevant section opf the pcb, showing mounting holes for C1: 5 / 7.5 / 10mm
and C4: 5 / 7.5mm:
An externally hosted image should be here but it was not working when we last tested it.
My best/only option for mounting 15mm dot pitch components would be to mount them on reverse side, but I guess that would be okay.
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