Gm = Transconductance = Forward Transfer Conductance
1 mho = 1 Siemens
2000 umho = 2 mS

Can 2mS to 6mS be considered high Gm? Compared to 22mS typical for 2SJ74BL that AMB stocks. Would 0.8mS be too low?

I will probably try matching a 2n5457/2n5460 combination or similar. But if all else fails will it be ok to just install BJTs without changing resistor values like the input resistor?

[Edited by Dave_M on 01-06-2007 at 05:26 PM.]

Lower distortion would be good. But measured performance isn't everything. Opamps would probably perform even better but I bet wouldnt sound as good as CKIII. Also cost is another reason.

I will try a couple of different JFETs, but at least I can fall back to BJTs if the JFETs are no good (I already know the all BJT version sounds very good from my prototype).

Sorry if I am being a nuisance. P.S. paid

[Edited by Dave_M on 01-07-2007 at 06:00 AM.]

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[Quoting Dave_M]

A few months ago I wanted to build kevin gilmore's dynalo but gave up on that idea because it was very difficult to get hold of the 2SK389 dual matched JFET. So I decided to only build amplifiers that used readily available parts.

Lower distortion would be good. But measured performance isn't everything. Opamps would probably perform even better but I bet wouldnt sound as good as CKIII. Also cost is another reason.

I will try a couple of different JFETs, but at least I can fall back to BJTs if the JFETs are no good (I already know the all BJT version sounds very good from my prototype).

Sorry if I am being a nuisance. P.S. paid

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Never a nuisance:d

There is an alternative to the 2sk389's?

Although i have a few 389 and 109's sitting around, looked at the gilmore but thought the 389's would do better in a phono stage.

allan

Dave_M

Just found this company in the UK http://www.grandata.co.uk/ that there website shows the JFETs in stock along with all the ones in Kevin Gilmores Dyno/Lite etc.

Regards

John

They have to go on the back burner for the moment cause there's another project ahead of it (as always). Nice job, AMB and Runeight, thanks

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[Quoting batman]

NOW THAT'S THE WAY TO RUN A GROUP BUY!!!! My boards arrived today less than a week after I paid for them. Nicely made boards too

They have to go on the back burner for the moment cause there's another project ahead of it (as always). Nice job, AMB and Runeight, thanks

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[Quoting batman]

NOW THAT'S THE WAY TO RUN A GROUP BUY!!!! My boards arrived today less than a week after I paid for them...

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Not to say that Ti's effort hasn't raised the bar for Group Buys in general, but it does help when you're in the same country as the person organising it

1. Be absolutely sure that all parts are installed in the proper orientation (electrolytic caps, diodes, transistors, voltage regulators, trimpots). Clean all flux residue from the board.
2. Before connecting the transformer, use your DMM to check for a short circuit between the 15VAC transformer input pads. Check for shorts between each supply rail to ground (use the vias marked "+15V" and "-15V", and ground is the big "bolt hole" in the center), as well as to each other. Also check for shorts between the OL and OR pads to OG.
3. Do not run the amp without the DC servo opamp installed. Since there is no global negative feedback, the DC servo is absolutely essential to keep the output DC offset in check.
4. Turn both trimpots to their fully counter-clockwise position before applying power for the first time.
5. Connect the transformer, making sure that you have the two secondaries wired in proper phase. If you have a volume pot connected to the inputs, turn it to minimum volume setting. Otherwise, short-circuit the left and right inputs.
6. Apply power. Check the voltage at the +15V and -15V pads relative to ground.
7. Check the output DC offset at each channel. They should both be no more than a mV or so.
8. Using your DMM in DC mV mode, measure the voltage across R23, and adjust trimpot R19 until you read approximately 8mV. This corresponds to 17mA through the output stage. Since BJTs have a positive temperature coefficient, the current will increase as the device warms up.
9. Repeat step 8 for the other channel (use R47 and trimpot R43).
10. Let the amp warm up for 15 minutes, re-check the voltages across R23 and R47, this time adjust for about 14mV voltage across them. This corresponds to 30mA through the output stage.
11. The output transistors should get slightly warm.
12. You may want to re-check the bias and re-adjust again.
13. Re-check the output DC offset.
14. If all is well, connect your source and headphones and enjoy the music.

One other thing: You can use a metal standoff at the PCB "ground hole" to connect the ground to the chassis. If your IEC AC inlet and power transformer is also in the same case, then you should also connect the AC earth ground to the chassis. This is how many commercial gear are wired up, but does create a possibility of ground loop if your source or other component in the audio chain also has its signal ground tied to AC earth. To work around this, you should isolate the PCB ground from the case and use a ground loop breaker between them.

[Edited by amb on 01-09-2007 at 03:24 PM.]

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[Quoting MASantos]

DOes anyone know if there is a wallwart that provides +-15V AC with enough current for this project? This would make casing much easier and clean and keep high voltages away from the hands of newbies.

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You need two independent 15V AC sources, and I have not seen such a thing in a single wallwart. But you could use two 15V AC wallwarts, rated at least 200mA each.

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[Quoting Mister X]

Assuming he means DC instead of AC....Elpac WM071-1950-D5? ;)
;)

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www.jameco.com/...roductId=115043

[Edited by batman on 01-09-2007 at 06:09 AM.]

MisterX, that is a nice solution, and one can omit the diode bridge and filte caps, saving some $ as well.

One can also buy a +-18v dc elpac or similar and still use the onboard voltage regulators.

I recently stumbled upon a local source that has these xfmr's for only $5 ea. (Austin Electronics - www.austinelex.com/ae_001.htm - but the guy is closing up his storefront in a couple of weeks.) I've been building up several for my SOHA's. You can use 6-32 standoffs or threaded spacers with 6-32 machine screws on either side to bolt the xfmr into the case, and close it up. As with your example, there's still plenty of room for the wiring and a line fuse.

After fooling around with DIN connectors, I just decided to run two line sets of 2.1mm plugs and use two 2.1mm sockets for connections to the board. Most of the time, you can find those line sets already made up or cut off of surplus wall adapters. This should easily work for the CKKIII, too. I'll try to post some photos this weekend.

BTW, I've also built a couple of TREAD's using an 800ma 24/48V $2.59 xfmr (also from Jameco) using those enclosures. There's just enough room at the bottom to fit the TREAD board. You can poke the LM317 outside through the back for mounting to a heat sink. If you put the power cap on the underside of the board and turn the whole thing upside down, you can get the trimmer's screw poking through the bottom of the walwart, too. Those little Jameco walwart enclosures are really versatile!! (sorry for the slight digression)

The amp is probably stable without C8, C9, C17, C18. I think they should be left out of the build. If the amp oscillates then you can put them in. The amp is faster without them.

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[Quoting seroxatmad]

Hi

Dave_M

Just found this company in the UK http://www.grandata.co.uk/ that there website shows the JFETs in stock along with all the ones in Kevin Gilmores Dyno/Lite etc.

Regards

John

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Hi
I'd wrote email to them with no any feedback.....are they still on business?

cheers

Coffin

Ref the 1455 hammond case.

In the UK i will be ordering from farnell - but find i can have an all black case (with either plastic or metal ends), or silver with silver ends.

On the hammond website they have pics of a silver case with black ends. This is what I fancy, i can then have a silver control knob and matching silver phone jack.

The problem is the separate black ends are sold as spares/options.

Anyone know where they can be bought as most suppliers do not seem to sell the options available for the cases.

John

P.S Amb do you have any pics of your unit wired and housed?

[Edited by seroxatmad on 01-10-2007 at 05:09 PM.]