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Old 12th March 2004, 08:45 AM   #1
Garbz is offline Garbz  Australia
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Default Hints for starting a KG amp.

Hey all

I've gotten quite adapt in making opamp based headphone amps recently. A heavily modified Cmoy serves my earbuds nicely while on the road, a simple and cheaply constructed PPA amp does quite nicely at home, but I think it's time to move on. I want to hear this Gillmore sound which is plastered accross every DIY amp forum on the internet.

Basically this will be my first solid state project and i just want some info so i don't spend money needlessly.

1. The pcb layout is on the site, i'll probably start working with that and expand it to include a power supply onboard and then make a PCB using the toner transfer method. No real question here.

2. Ok question. I've seen lots of picks of various Gillmore amps. All of them have low-Z headphones attached. I have Sennheiser hd580s which are rather the opposite. Do i need to modify the original design? How do i modify the original desgin? Or do i need to take my poor headphones to a different amp.

3. I've read everywhere about the importance of matching the parts. So starting with resistors is easy since they normally come in a pack of 8 or so and resistors are relatively cheap. Just run the multimetre over them to measure the impedance i suppose?

4. About transistors, do i need to do that too? How important is it? Transistors are relatively expensive and i don't have a ready supply that i can simply swap out a few that are slightly off. Or do i only need to match each resistor to it's counterpart on the other side?

5. My multimetre has a yet unused transistor checking thingy (i lost the manual). It spits out a number in the hundreds when a transistor is pluged in. Can i just try and closely match the numbers in that or do i need to use that little circuit on the headwize guide.

Finally anything else i should know? Any part of the amp i should be perticularly carefull with?

Thanks for any help in advance.
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Old 12th March 2004, 11:46 PM   #2
Nisbeth is offline Nisbeth  Denmark
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2) You could alter the gain, but get the circuit working first before you start modding anything. If look over the threads at head-fi and headwize, several people have had serious difficulties getting this to work right (yours truly included )

3) You basically need to match everything in the circuit. Do the resistors like you describe.If you buy 1% resistors you probably need to buy ~50% extra to get good matches.

4) Definitely yes! Use the method described in the article at headwize, that's the easiest. You probably need at least 30 each of the 2SC1x15 to get decent matching pairs. 50 is better.

5) You could, but I'd recommed using the method described in Gilmore's article.


As far is I am able to tell, the LED's are a critical point of the circuit. Get extras of those as well. You can use sockets in the LED positions so you can experiment with the combinations that give you the lowest DC-offset (give the amp plenty of time to heat up between your changes so the offset gets a chance to stabilise). I started out using an OPA2134 for the servo opamp. My offset-problems were reduced immediately when I exchanged it for an OP27 so you might want to start out with one of those (or an OPA227).
This is not "mission impossible", but it is a lot more difficult than a Cmoy or a PPA.
I recommend that you search the archives at headwize and head-fi (for Gilmore and KGCA) and read what else has been posted about building and troubleshooting this amp.


/U.
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Old 13th March 2004, 01:46 AM   #3
Garbz is offline Garbz  Australia
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50 Transistors !!!! (looks into wallet and cries). Oh well in war there must be casualties.

I'm aware that this isn't a simple opamp. I believe i have to tools and the resources to get this running though, and if i do from what i've read it will be more than well worth it. Don't get me wrong the CMOY was kinda nice, the PPA much nicer, but this seems to be one of the holy grails of DIY discreet designs.

LEDs aren't a problem i think i even have a 50 pack somewhere, and they are cheap too.
Resistors. I dunno but it should be fine.
On an upside buying the transistors in large lots should bring the price down.

OP27 will be hard to source since i'm from Brisbane Australia and we have nothing decent here store wise, except for Rs-components. I can see a rather large order comming in from tiwan

p.s. The resources mentioned in the above post include this forum so expect me back
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Old 14th March 2004, 10:31 AM   #4
Garbz is offline Garbz  Australia
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Hey all i have a question regarding testing.

I'm going to have problems sourceing parts for the power supply for a while so the question is simply:

How can i test the amplifier itself? All i have is a 12v regulated wallwart. Will the amp run of a ... lot of batteries? Does the amp require exactly 32.8v (16.4 + / - )? Is it too much to ask for this thing to produce sound at 12v (6+/-)?
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