Joe Rasmussen "Trans-Amp" - 40 Watt Transconductance "Current Amplifier" - Page 6 - diyAudio
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Old 6th June 2014, 11:22 PM   #51
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Quote:
Originally Posted by Jaxhouse View Post
Joe, just found this current domain amp schematic and is there a parts estimate build cost for this excluding the case ?
Would it also work with planar magnetic headphones 50/60 Ohm ?
Haven't done a parts estimate build, but this is not an expensive build. As for compatibility with headphones, that I cannot say for sure, what impedance are they and have the published an impedance plot? Planars tend to have flat Z, a good thing. I would guess that a 15 Ohm 10W resistor on the output of the amplifier, would make many headphones compatible, and even tweak that resistor by ear and get away with the highest value possible. At least, it is less likely that this amp does any damage to the headphones in a way that many speakers could. So it is rather safer and I suspect this would make a rather above the norm quality wise headphone amp.

Cheers, Joe
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Old 7th June 2014, 01:55 AM   #52
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Quote:
Originally Posted by Joe Rasmussen View Post
Haven't done a parts estimate build, but this is not an expensive build. As for compatibility with headphones, that I cannot say for sure, what impedance are they and have the published an impedance plot? Planars tend to have flat Z, a good thing. I would guess that a 15 Ohm 10W resistor on the output of the amplifier, would make many headphones compatible, and even tweak that resistor by ear and get away with the highest value possible. At least, it is less likely that this amp does any damage to the headphones in a way that many speakers could. So it is rather safer and I suspect this would make a rather above the norm quality wise headphone amp.

Cheers, Joe

FostexT50RP 50ohm/LCD-2 60 ohm flat impedence.

LCD-2 maybe best with 3watts of headroom per ear .....so 4-6 watts max total output.

Well it is good to know that it may be possible as current domain headphone amps are few and far between, thus prices of $1800-$3000 which is priced on lack of competition more than exotic internals I'd guess.
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Old 17th June 2014, 06:19 PM   #53
adason is offline adason  United States
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Hi Joe, any opinion about this circuit of current amp? I got plenty of LM1875, if it looks promissing, I may go ahead an built it, just for fun.
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Old 17th June 2014, 10:58 PM   #54
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Quote:
Originally Posted by adason View Post
Hi Joe, any opinion about this circuit of current amp? I got plenty of LM1875, if it looks promising, I may go ahead an built it, just for fun.
Why not have some fun then? That circuit, wonder what the DC levels would be like if copied just as shown. But a trimpot to the negative rail may be needed. That 220R on the output will hep when no load or go really high, but with load, needs to be trimmed.

Cheers, Joe
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Old 8th August 2014, 02:22 PM   #55
adason is offline adason  United States
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Just want to report...I built the circuit above, and not only its works fine just like it is, it sounds great. While normal recommended LM1875 circuit is ok, inverting with buffer sounds better. However, this transconductance circuit beats them all. It sounds liquid, like good tube amp.
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Old 11th August 2014, 08:40 PM   #56
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Quote:
Originally Posted by adason View Post
Just want to report...I built the circuit above, and not only its works fine just like it is, it sounds great. While normal recommended LM1875 circuit is ok, inverting with buffer sounds better. However, this transconductance circuit beats them all. It sounds liquid, like good tube amp.
Well done.

Yup! It surprised even a tube guy like me.

Cheers, Joe
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Old 11th August 2014, 09:19 PM   #57
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Nice project.
Greetings
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Old 12th August 2014, 08:51 PM   #58
Zigis is offline Zigis  Latvia
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Really interesting amps!

adason, are my calculation of gain of your amp right 22k/0.25+1 = 89 ???

O no, I just notice, R7 is 0.25 ohm, not K. How large is gain and how to calculate it?

Where you place R6 - close to chip or on speaker terminals?
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Old 12th August 2014, 08:57 PM   #59
adason is offline adason  United States
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all parts are as close to the chip as I manage
I have no idea how to calculate the gain, sorry
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Old 12th August 2014, 09:28 PM   #60
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Quote:
Originally Posted by Zigis View Post
Really interesting amps!

adason, are my calculation of gain of your amp right 22k/0.25+1 = 89 ???

O no, I just notice, R7 is 0.25 ohm, not K. How large is gain and how to calculate it?

Where you place R6 - close to chip or on speaker terminals?
You can see a discussion of current source op amp configurations here:
Variable Amplifier Impedance

The gain depends on the load impedance.

No current flows into/out of the + terminal so the 22k resistor will not affect the gain.
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