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Old 28th March 2011, 06:18 PM   #1011
qusp is offline qusp  Australia
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Quote:
Originally Posted by Fenris View Post
The "average" audio op-amp used for most outputs like the 2132 or 627 have about 20ma of current capability with a 600 ohm load. Two and a half times the current capability will give five times the output power and the overall output impedance will be around 10 ohms. This probably isn't the "ideal" or "ultimate" solution, but I'm willing to try something unusual and unique. I could drop the 1014 in there, but I've already got enough 2sk170s that need a good home.
and neither of those chips would go anywhere near satisfactorily powering my hd600 directly, not that i dont like the right opamps in the right design either, but both of those would need at the very least a high current buffer like buf634 or lme49600/10, a mosfet follower. etc and into lower ohms you will get considerably less power and bass will suffer big time

sure mate give it a go, sorry i'm really not trying to rain on your parade, i like seeing headphone users here on the forum, not enough of us. It just seems like a lot of effort to get what i would consider to be pretty subpar results with your headphones

and yes i'm a bad headphone geek. no reason you cant use those jfets, but you'll need more than 5 imo, a further gain stage, or current buffer; possibly some of EUVL's to92 heatsinks to crank a bit more juice out of the jfets??
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Old 28th March 2011, 06:31 PM   #1012
Fenris is offline Fenris  United States
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When I get the prototype PCB back from the board house I'll populate it and let you know the results. I've got a mix of matched G and B 2SK170s, so I should be able to get at least 30 ma per channel. If it's still not enough, I'll have to try bigger JFETs or small FETs.
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Old 28th March 2011, 07:14 PM   #1013
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sure, if youde rather keep it out of the thread shoot me a pm, pretty sure i've got a couple parts here i'de be willing to donate to a good cause; lovoltechs etc. i like a happy ending as much as the next guy.
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Old 25th April 2011, 08:04 PM   #1014
ramallo is offline ramallo  Europe
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I bought a pair of chinese pcbs from eBay (Very close to Manu's board), and I did a jfet boz but have a excessive gain for my system. I merged data of different post of this thread (And some others) and I made this new one, Anything wrong?

Click the image to open in full size.
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Last edited by ramallo; 25th April 2011 at 08:08 PM. Reason: Photo
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Old 25th April 2011, 11:23 PM   #1015
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You could have dropped the gain by increasing the source resistor. I have read where it is suggested the source resistor should be 20 ohms anyhow. This is local feed back so not as bad as global.
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Old 26th April 2011, 04:24 AM   #1016
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Your 25K inuput resistor is too high. As it stands, your feedback current will overwhelm your input giving you alot less than the 2x gain you're looking for. The current will flow from the feedback through the volume pot since it is lower resistance the the 1M ground reference resistor. This will also make a variable gain circuit, with gain reducing as you attempt to increase volume. I know because I built one (see 12 posts above). I ended up putting a 4.7K resistor to ground instead of 1M, but that affects the input impedance. My recommendation would be to increase the series input resistor and feedback resistor to about 10x or 20x what they are currently.
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Old 26th April 2011, 09:24 AM   #1017
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I'll try with the source resistor

I have finished the preamp of the schematic, and actually I measured 4,48dB of gain at full pot, and a maximum of 2V RMS at output (1%). But I'll try too with the higher resistors.

Thanks!!!
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Old 26th April 2011, 10:22 AM   #1018
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The type of resistors at position R7, R13 will have a direct effect on the sound so try a variety - metal, carbon, tantalum, Zfoils, etc - with these simple ccts, everything has an effect, inluding the current through the jfets, naturally.

Suggest reduce C5, C6 to about 220uF (another critical device) and can reduce C8 to about 4u7 and try Mundorf Supreme, etc.

... 2 cents
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Old 28th April 2011, 02:08 PM   #1019
ramallo is offline ramallo  Europe
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Quote:
Originally Posted by mhouston View Post
You could have dropped the gain by increasing the source resistor. I have read where it is suggested the source resistor should be 20 ohms anyhow. This is local feed back so not as bad as global.
I tried with a 26,3 Ohm resistors and now the gain is 23,3dB, just dropped a bit, but isn't enough for my system. I'll try raise the value of these resistors.

In other hand, I think that for get more voltage swing at output the value of R12 must be increase and the value of R7 must be decrease, looks that both resistors acts like a tension divider. I think that R7 will be of 1K and R12=5K or more, am I on a mistake?.

Cheers
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Last edited by ramallo; 28th April 2011 at 02:19 PM. Reason: more data
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Old 28th April 2011, 02:29 PM   #1020
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Quote:
Originally Posted by jameshillj View Post
The type of resistors at position R7, R13 will have a direct effect on the sound so try a variety - metal, carbon, tantalum, Zfoils, etc - with these simple ccts, everything has an effect, inluding the current through the jfets, naturally.

Suggest reduce C5, C6 to about 220uF (another critical device) and can reduce C8 to about 4u7 and try Mundorf Supreme, etc.

... 2 cents
I'm using Vishay 1% 0,6W metal resistors, I tried with a Takman's carbon resistors at R11 and I didn't like the sound (Probably best with the metal ones). For capacitors I'm using Solen.

I added at C5 and C6 positions a small film capacitor in parallel (100nF)

P.S. In a B1 I used a 4u7 Mundorf silver in oil at output and I'm very happy with the result
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