Marsh headphone amp from Linear Audio

[Telstar]

Originally Posted by jackinnj

I've put together some kits of parts for the Marsh headphone amplifier -- you've got two choices: 1) Semiconductors and active components = $54;
2) Semiconductors = $27.50.

What's missing in the semi only kit? Caps and resistors?

[Scott Stansbury]

Just confirming the power ratings on the resistors - the board seems sized for 1/8 W and 1/4 W rather than the 1/4 W and 1/2 W listed in the BOM...

Thanks!

[jackinnj]

The CMF55's may be 1/8W -- it depends.

Dick put a 100nF film cap in parallel with the 220uF -- i can include these as well.

The semi's only includes the diodes, jfets, 5087/8's and output transistors.

[Scott Stansbury]

I'm interested in the "semi's only" kit. Should we wait for the web page at your site - or can you send a PayPal invoice directly?

Thanks for your effort in this!

[jackinnj]

I will put the items on the webstore on Wednesday night. Will change the shipping on all items to $6 for the US, $7 to Canada and $9 for Europe.

[jackinnj]

Well, that was fast -- i now need to get more boards burned! You guys cleaned me out.

[RNMarsh]

Now if you want to hyper tune the amp, and if you can measure the harmonic distortion way down low ---- You can add another trim in place of R10... a value of 500 ohm should work well. Or, you can jumper in a 2K pot across the existing R10 and adjust for distortion null and replace pot with a bridging R value of the trim pot value. I found some more reduction in thd this way when under load (<100 Ohms).

Here is my proceedure to try if you have the thd equipment: Apply load R to the amp.... using the dc offset pot, trim to minimum thd. You will then have a dc offset. Then, using the pot at R10, readjust for zero dc offset. Maybe a little back and forth and you will have the lowest possible thd... even beyond matching. In my case changing R10 value by only 40 ohms improved the performance under heavy loads. So, use a multi-turn trimming pot. How much improvement depends on how well you matched all your transistors. The matching of the jFET provided by jackinnj is even tighter than i did... so i cant say how much better you can get under load, but its what i do.

Thx- Richard

[Telstar]

BTW Richard, I want to thank you for the suggestion of the Peak tester in the article. When I re-read it last week I was just looking for a new tester.
I got the 75 model mostly because of the wider range and basic jfet testing. For small signal transistors is very quick and works for LEDs too

[RNMarsh]

Originally Posted by Telstar

BTW Richard, I want to thank you for the suggestion of the Peak tester in the article. When I re-read it last week I was just looking for a new tester.
I got the 75 model mostly because of the wider range and basic jfet testing. For small signal transistors is very quick and works for LEDs too

There are quit a few other interesting test equip out of Taiwan and China at attractive prices; Curve tracers are avail. Also, this one --> Another portable semiconductor tester that also tests 78xx and 79xx 3-term regulators and power transistors (2A and 1Kv !) etc. Model DY294 made by DUOYI Elect.
-RNM

[RNMarsh]

Originally Posted by RNMarsh

There are quit a few other interesting test equip out of Taiwan and China at attractive prices; Curve tracers are avail. Also, this one --> Another portable semiconductor tester that also tests 78xx and 79xx 3-term regulators and power transistors (2A and 1Kv !) etc. Model DY294 made by DUOYI Elect.
-RNM

The new PEAK Atlas 75 Pro Telstar mentions just came out this Dec 2012. About $185 in USA. With a PC does curve tracing, too... Nice. -Thx RNmarsh