Marsh headphone amp from Linear Audio

OK, I simmed it. I don't have the types of transistors so I just guessed. It's a good circuit, but kind of strange. As I understand it, the first two transistors in each half (J1, Q1 and J2, Q2)are both set up as gain stages with 100% feedback. The output stage (Q3, Q4) has constant current to each emitter resistor via Q6 and Q7. The output stage transistors seem to be operating as phase splitters though, since a voltage will be developed across the collector resistors R8, R9. I suppose Q3, Q7 and Q4, Q6 operate together to keep constant current in the emitter resistors. Pot P1 provides a feedback path to the input stage.

I measured -3dB of gain with the transistors I used. Distortion was obviously extremely low. Bandwidth had a bump in the 10MHz region, but that's a function of the particular transistors.
 
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OK, I simmed it. I don't have the types of transistors so I just guessed. It's a good circuit, but kind of strange. As I understand it, the first two transistors in each half (J1, Q1 and J2, Q2)are both set up as gain stages with 100% feedback. The output stage (Q3, Q4) has constant current to each emitter resistor via Q6 and Q7. The output stage transistors seem to be operating as phase splitters though, since a voltage will be developed across the collector resistors R8, R9. I suppose Q3, Q7 and Q4, Q6 operate together to keep constant current in the emitter resistors. Pot P1 provides a feedback path to the input stage.

I measured -3dB of gain with the transistors I used. Distortion was obviously extremely low. Bandwidth had a bump in the 10MHz region, but that's a function of the particular transistors.

I dont think you have the circuit quite right just yet, the -3db gain doesnt sound right. There is more than one way to go about giving the shown circuit gain, depending on how much gain needed.
 
What's the output level? And are the values in the notes field the load impedance?

se

For that FFT I used 250R == since my Sennehiser's are 300 ohms with 1 volt out. That should be over 100dB on the HD-650 cans. Dick stated that he ran them up to 12 Watts out into 8 ohms, but I think those TO-126 devices would be perspiring pretty badly at that level! I think the amp would go well with something like the Grado SR60's.

The 2 resistor values in the chart are the source resistors on the JFET's. Dick indicated that he used 51.1 ohms. The devices were NOT matched for that particular run. Note that in the article the designators R3 and R4 were accidentally used twice.

I set up the other side substituting matched 2SB649AC/2SD669AC for the MJE (On Semi) devices. Hfe is ~155, Ft is 140MHz for these. The THD is lower, but the noise level is higher.
 
I doubt that is the complete schematic, -3db gain means it has no voltage amplification. That would make the circuit virtually useless. Have a look at D Self series about CFP with gain, then Id say your on the right path to discover how the circuit works.

Uh yeah right. If you can't explain then I'm sure as hell not going to go find some book about it. Who do you think you are anyway? If you can't explain it then you don't understand it either. Talk about a useless post.

Look, a zero dB circuit is just fine since the standard output for CD players is 2Vrms, which will make you deaf while wearing headphones.
 
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For that FFT I used 250R == since my Sennehiser's are 300 ohms with 1 volt out. That should be over 100dB on the HD-650 cans. Dick stated that he ran them up to 12 Watts out into 8 ohms, but I think those TO-126 devices would be perspiring pretty badly at that level! I think the amp would go well with something like the Grado SR60's.

The 2 resistor values in the chart are the source resistors on the JFET's. Dick indicated that he used 51.1 ohms. The devices were NOT matched for that particular run. Note that in the article the designators R3 and R4 were accidentally used twice.

I set up the other side substituting matched 2SB649AC/2SD669AC for the MJE (On Semi) devices. Hfe is ~155, Ft is 140MHz for these. The THD is lower, but the noise level is higher.

That is a bit dissapointing, Jlh s headphone amp with just 6 transistors can do that and better depending on how hard you bias the outputs.
 
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Uh yeah right. If you can't explain then I'm sure as hell not going to go find some book about it. Who do you think you are anyway? If you can't explain it then you don't understand it either. Talk about a useless post.

Look, a zero dB circuit is just fine since the standard output for CD players is 2Vrms, which will make you deaf while wearing headphones.

That depends on your headphone impedance.

Please be polite, I dont want to say too much otherwise Ill have the designer and linear audio on my back, theres an obvious money trail going about. :(
It will have to be upto them to disclose the circuit if they want, for others that dont understand it I can at max give a hint or two. You dont need his book, this stuff is available on the internet.
 
That depends on your headphone impedance.

Please be polite, I dont want to say too much otherwise Ill have the designer and linear audio on my back, theres an obvious money trail going about. :(
It will have to be upto them to disclose the circuit if they want, for others that dont understand it I can at max give a hint or two. You dont need his book, this stuff is available on the internet.

Thanks, but I have my own designs that I like plenty good enough. I've been doing my own spice models for a little while now and developed some circuits I think are pretty cool. I don't need to read about someone else's "magic" circuit. I've already figured out that the output stage in this Marsh headphone amp isn't that great, for example.
 
Simon B

I think Linear Audio does provide benefit for this DIY forum (even for those who dont buy the magazine).
I suspect the motivation behind the volumes is more to keep interest in DIY audio than profit. The books are well made, the authors seem to be for the most part members here and it is clear from the articles that months of work went into them (let alone the fact of knowing that at least this forum will try to dismantle much of what they show). There are no advertisements, and claims are backed up with analysis or measurements.

I love the DIY 3D layout and similarly the price seems to negate any profit.

Commercialism didnt even enter my mind until I saw your post. I can understand your view, but having enjoyed all the volumes to date and seeing the overall benefit to this DIY forum I glad to see this thread.

Hope this helps
-Antonio
 
Figure 3a basically shows the output stage. He doesn't like it.

Devil's in the details. RM's circuit runs in class A, the figure you reference doesn't. IMO, class A is no big deal, but I can't argue with RM's results, the measurements Jack showed were outstanding.

I've got no dog in this hunt, I hardly ever use headphones, but the article was educational, and it's interesting to understand the thought process that an experienced and accomplished designer uses. No doubt others are far beyond my abilities and this is all old hat to them, but I'm just a dumb chemist.
 
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Simon B

The books are well made, the authors seem to be for the most part members here and it is clear from the articles that months of work went into them (let alone the fact of knowing that at least this forum will try to dismantle much of what they show). -Antonio

For several 100's of hours of work, this time I got myself a used mint Thorens 160A with a Shure V15 (before the wife spent it).