Marsh headphone amp from Linear Audio

[Steve Eddy]

Originally Posted by jackinnj

Now, here's a real FFT run with a real measurement instrument (not some POS soundcard, an Audio Precision SYS2722) to demonstrate what I experienced.

What's the output level? And are the values in the notes field the load impedance?

se

[dirkwright]

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.

[homemodder]

Originally Posted by dirkwright

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.

[dirkwright]

Originally Posted by homemodder

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.

OK, so how much gain is it supposed to have, as shown in the schematic posted earlier in this thread?

[jackinnj]

Originally Posted by Steve Eddy

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.

[homemodder]

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.

[dirkwright]

Originally Posted by homemodder

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.

[homemodder]

Originally Posted by jackinnj

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.

[SY]

Dirk, the article on the headphone amp is well worth reading. The schematic posted here is the unity gain buffer section. The overall design has 12dB of gain.

[homemodder]

Originally Posted by dirkwright

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.