richrips Posted January 31, 2010 Share Posted January 31, 2010 i'm trying to build a circuit to boost the gain on the preamp section of my ashdown little giant 1000. [url="http://klaas.homeip.net/wb/media/pdf/Ash_Giant_Buffer.pdf"]http://klaas.homeip.net/wb/media/pdf/Ash_Giant_Buffer.pdf[/url] i've not made a circuit recently and am not sure which capacitors to use. maplin sell several different types with different voltage ratings. what is this voltage rating and which ones should i use for the circuit? does anybody have experience in this type of thing? apparently the circuit is based on a standard volume boost pedal. any help much appreciated, cheers, rich Quote Link to comment Share on other sites More sharing options...
Bloodaxe Posted January 31, 2010 Share Posted January 31, 2010 [quote name='richrips' post='730237' date='Jan 31 2010, 01:23 AM']i'm trying to build a circuit to boost the gain on the preamp section of my ashdown little giant 1000. [url="http://klaas.homeip.net/wb/media/pdf/Ash_Giant_Buffer.pdf"]http://klaas.homeip.net/wb/media/pdf/Ash_Giant_Buffer.pdf[/url] i've not made a circuit recently and am not sure which capacitors to use. maplin sell several different types with different voltage ratings. what is this voltage rating and which ones should i use for the circuit? does anybody have experience in this type of thing? apparently the circuit is based on a standard volume boost pedal. any help much appreciated, cheers, rich[/quote] The voltage rating is the nominal maximum voltage that the capacitor can withstand before the dielectric fails & the Magic Smoke escapes. In the circuit you've posted the only one that it would really apply to would be the electrolytic at C4. Anything around or over 25V should be plenty. I'm bemused by that circuit... Why have a 0.22uf cap on the input side & a 10uF on the output? Surely it should be 10uF on both sides? I've got to ask... Why are you trying to do this? Is the Ashdown not loud enough? Or have I completely misread this & you're after building a booster stomp box? Pete. Quote Link to comment Share on other sites More sharing options...
richrips Posted January 31, 2010 Author Share Posted January 31, 2010 [quote name='Bloodaxe' post='730263' date='Jan 31 2010, 03:05 AM']The voltage rating is the nominal maximum voltage that the capacitor can withstand before the dielectric fails & the Magic Smoke escapes. In the circuit you've posted the only one that it would really apply to would be the electrolytic at C4. Anything around or over 25V should be plenty. I'm bemused by that circuit... Why have a 0.22uf cap on the input side & a 10uF on the output? Surely it should be 10uF on both sides? I've got to ask... Why are you trying to do this? Is the Ashdown not loud enough? Or have I completely misread this & you're after building a booster stomp box? Pete.[/quote] Hi pete, the ashdown has a weak preamp, which has been pointed out by various other people here and on other forums/reviews. i can't get the input level indicator up to halfway, even with the pre-gain maxxed out completely. This guy made this circuit to boost the gain on his lg350, which has the same preamp, so i should be able to use the same schematic to boost my lg1000. i think (according to google translator) he built it in to the effects loop. [url="http://klaas.homeip.net/wb/pages/diy/amps/ashdown-little-giant-350.php"]http://klaas.homeip.net/wb/pages/diy/amps/...e-giant-350.php[/url] i like the idea of not having a seperate stompbox as this keeps everything neat and portable with no need for extra power supplies, pedals, cables etc. do you think his circuit would work? i'm assuming it worked for him? i don't really know how the circuit works so can't comment on which cap should go where. any advice much appreciated. cheers, rich Quote Link to comment Share on other sites More sharing options...
AlanP2008 Posted January 31, 2010 Share Posted January 31, 2010 The circuit should work. The volume control will give a gain change from about unity gain to around x10 (20dB). The 10uF electrolytic should be 30V or more. It doesn't say, but the volts at K1 shoud be a good clean DC supply of +12V to say +32V or so, whatever is available in the amp. The issue about capacitor values is that the cap forms a high-pass filter with the resistance to ground that follows it. On the input side, that resistance is known, being effectively the 470k resistor... so 0.22uF is plenty big enough. On the output side, the load inpedance isn't known, as it will be whatever impedance presented by the thing the amp is driving, so it makes sense to make the coupling cap big enough to ensure a good bass response for any sensible low values of load resistance. Hope this helps. Alan Quote Link to comment Share on other sites More sharing options...
Bloodaxe Posted January 31, 2010 Share Posted January 31, 2010 [quote name='AlanP2008' post='730708' date='Jan 31 2010, 04:02 PM']The circuit should work. The volume control will give a gain change from about unity gain to around x10 (20dB). The 10uF electrolytic should be 30V or more. It doesn't say, but the volts at K1 shoud be a good clean DC supply of +12V to say +32V or so, whatever is available in the amp. The issue about capacitor values is that the cap forms a high-pass filter with the resistance to ground that follows it. On the input side, that resistance is known, being effectively the 470k resistor... so 0.22uF is plenty big enough. On the output side, the load inpedance isn't known, as it will be whatever impedance presented by the thing the amp is driving, so it makes sense to make the coupling cap big enough to ensure a good bass response for any sensible low values of load resistance. Hope this helps. Alan[/quote] Ah, that makes sense. Thanks for that (it's been a long time since my C&G & I can remember sufficient circuit theory to be dangerous!) Pete. Quote Link to comment Share on other sites More sharing options...
jensenmann Posted January 31, 2010 Share Posted January 31, 2010 When you buy parts make sure that they fit on your PCB. I´d go for an OPA134 instead of TL071. Less noise, less distortion and cleaner sound, though it has double supply current. There´s a bandwidth limitation cap in the circuit missing. Put a 10pF NPO/C0G ceramic cap across the pot. This should be enough (roughly guessing) Quote Link to comment Share on other sites More sharing options...
richrips Posted January 31, 2010 Author Share Posted January 31, 2010 Am i correct in thinking that R6 is a variable resistor (i.e. a potentiometer or pot) on the diagram? i can't really see anything variable on the photographs. it looks like a fixed value standard resistor in the photos (i'm thinking its the one that sticks up rather than lying flat like R1, R2 etc). should i use a 100k resistor here? also, is anyone a good german translator? i'm trying to work out where in the signal path the circuit goes. will i see increased gain on the visual meter or will it just be louder with no change in the display level? i may have a few more questions when i come to install it. thanks very much for the help so far. once again impressed by the wealth of knowledge out there! cheers, rich Quote Link to comment Share on other sites More sharing options...
jensenmann Posted January 31, 2010 Share Posted January 31, 2010 drop me a pm with the text you need to get translated. Quote Link to comment Share on other sites More sharing options...
AlanP2008 Posted February 1, 2010 Share Posted February 1, 2010 (edited) [quote name='richrips' post='731127' date='Jan 31 2010, 09:56 PM']Am i correct in thinking that R6 is a variable resistor (i.e. a potentiometer or pot) on the diagram? i can't really see anything variable on the photographs. it looks like a fixed value standard resistor in the photos (i'm thinking its the one that sticks up rather than lying flat like R1, R2 etc). should i use a 100k resistor here? <snip> i'm trying to work out where in the signal path the circuit goes. will i see increased gain on the visual meter or will it just be louder with no change in the display level? cheers, rich[/quote] It is shown as a pot (variable resistor), but unless you have front-panel space for it, you could use what is known as a preset pot - which is a small device where you would set the resistance with a screw-driver... When the resistance is lowest, the gain will be lowest, and vice-versa. Alternatively, you could fit a 100k fixed resistor, and if it gives you too much gain, replace it with 47k or 33k (probably not too much point going lower than that....) You could hijack the instrument input, and put this amp right at the beginning, however, it might be a bit noisy in that location. Alternatively, you are likely to be able to break into the signal path in the region of the send/return or line-in jack. If you have an external power supply (even 2xPP3 batteries), you could insert the amp into the signal chain from outside the amp using the send/return point for testing (I recommend you do this)... On the subject of recommendations, if it were me (and I can buld this stuff myself, as you can imagine...) I'd be using some kind of stomp box to provide the amplification... The Danelectro Driive pedal is as cheap as chips, does nothing at all to the sound if you turn down the "drive" level, and has an output "level" control that will provide plenty of gain... Or a Boss CS-3 compression pedal... or an EQ pedal... Basically this approach is fool-proof, and avoids modding the amp itself... (On the amp modding subject, I'd be surprised if a single resistor change somewhere in the existing amp circuit wouldn't get the amp itself to give you the gain you want - if you have a schematic, I could figure out where... but then, why didn't Ashdown get the gain structure right to start with... everyone seems to complain about low gain on these amps...) Hope this helps. Alan edit: Just spotted I don't like the value of C1 - I'd put a much bigger cap in there 10uF like C4 would be fine - +ve end upwards. Edited February 1, 2010 by AlanP2008 Quote Link to comment Share on other sites More sharing options...
jensenmann Posted February 1, 2010 Share Posted February 1, 2010 Finally I´d recommend a booster pedal, too. Snipping wires in your amp is not funny unless you´re expierienced in DIY-electronic. C1 is theoretically ok. I´d stay with a film cap for sound (less THD than electrolytics) reason but I´d make it as big as possible. Limit is money and space on the PCB. It´d probably be a Wima MKS-2 1uF/63V. Quote Link to comment Share on other sites More sharing options...
AlanP2008 Posted February 1, 2010 Share Posted February 1, 2010 [quote name='jensenmann' post='731751' date='Feb 1 2010, 02:28 PM']Finally I´d recommend a booster pedal, too. Snipping wires in your amp is not funny unless you´re expierienced in DIY-electronic. C1 is theoretically ok. I´d stay with a film cap for sound (less THD than electrolytics) reason but I´d make it as big as possible. Limit is money and space on the PCB. It´d probably be a Wima MKS-2 1uF/63V.[/quote] Unless I'm doing this wrong (and I'll admit that it has been a long time since I did this stuff professionally)... my back-of-a-fag-packet calculation of the -3dB point for a 10k resistor (R4) and a 0.22uF capacitor (C1) is about 75Hz... that's where the gain of the op-amp will start to roll off towards unity.... Not low enough I would think... 1uF would be about the smallest you would want - 0.22 is way too small... Quote Link to comment Share on other sites More sharing options...
jensenmann Posted February 1, 2010 Share Posted February 1, 2010 [quote name='AlanP2008' post='731849' date='Feb 1 2010, 04:56 PM']Unless I'm doing this wrong (and I'll admit that it has been a long time since I did this stuff professionally)... my back-of-a-fag-packet calculation of the -3dB point for a 10k resistor (R4) and a 0.22uF capacitor (C1) is about 75Hz... that's where the gain of the op-amp will start to roll off towards unity.... Not low enough I would think... 1uF would be about the smallest you would want - 0.22 is way too small...[/quote] The source sees a voltage divider formed by C3 and R5. IC is connected between these two with 10TOhm input resistance, virtually infinity which has no influence on the -3dB frequency. It´s an noninverting topology, R4 only references the - input to virtual ground= half V+ since the circuit runs from single rail. R4 has nothing to do with the input impedance hence no influence on -3dB point. Quote Link to comment Share on other sites More sharing options...
AlanP2008 Posted February 1, 2010 Share Posted February 1, 2010 (edited) I'm not talking about the input impedance of the op-amp, it is the gain configuration that is important... If the Vref point were an effective ground, then the (non-inverting) gain of the opamp circuit would be set to (R6 + R4)/R4. However, Vref is not a particularly good ground - it is decoupled to actual ground by 0.22uF, so that capacitance appears in series with R4... At about 75Hz, that 0.22uF cap shows roughly 10k (the same as R4), which roughly halves the gain of the circuit (R6 + R4 +R4)/(R4 + R4) Ok, R1 and R2 also appear in parallel with C1 from an AC standpoint, which will make a bit of difference difference... but the principle stands... See my point? Edited February 1, 2010 by AlanP2008 Quote Link to comment Share on other sites More sharing options...
richrips Posted February 2, 2010 Author Share Posted February 2, 2010 I'm actually in contact with Mark and Chris at Ashdown who are being really helpful with this issue so that may help too. i'll forward them all suggestions thus made. cheers, rich Quote Link to comment Share on other sites More sharing options...
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