[mgalusha]

Josh,

While I'm certainly no expert, the feedback loop is from the output, through the C4/R18 pair and into the bases of Q2/Q4. I think Greg mentioned something like 80dB of gain in the differential input stage plus another 30dB in the output, so there has to be a pretty fair amount of feedback to knock this down and I am guessing it couldn't be altered much without causing problems.

'Course this is just a semi-educated guess, so it may be totally wrong.    

Hopefully Greg will provide the correct answer. Also, I am curious as to the Zin.

mike

[buckeyefanandy]

What will the price be post introduction?

[amplifierguru]

Thanks guys,

Yes there is a lot of feedback - with some 80dB gain in the first stage (best place before PS intrusion) and a further 30dB from the output stage. With typ closed loop gain of 30dB this leaves 80dB feedback. Needless to say damping factor is unmeasurably high and THD at 12.5W (my soundcard loopback) is typ 0.003% 2HD rising to 0.025% at 100W of predom 2HD and 3HD (higher hamonics -10dB max) for a very clean and transparent sound.

How's it sound, from one of the first owners -

http://www.diyaudio.com/forums/showthread.php?postid=827293#post827293

http://www.audioheritage.org/vbulletin/showthread.php?s=8564b75417efb6d8ba91b3e3bc6003a6&t=9021

Hi buckeyefanandy,

A$200 (roughly US$150) a pair SKA (GB150D) kits after the price rise. Love your postscript. Agree absolutely.

Hi mgalusha,

Received your order - thanks! Your post highlights one great application for these compact amps - rebirthing a gutted old amp chassis utilizing the chassis, heatsinks and power supply. An excellent resource friendly economy.

Cheers,
Greg

[tigzstudio]

do you plan on offering a fully assembled version in the future with heatsink/power supply?

[ctviggen]

For $100, how could you go wrong?  Is it really $100?  Can you even source the parts for that?  I might have to pony up for this just to see what it's like.

[amplifierguru]

Hi tigzstudio,

No fully built amps in chassis as yet - I'm still trying to locate a good and reasonable metalshop! as soon as i do the modules will be available as monoblocks and stereo units possibly a Home theatre 6 channel down the track, all as kits or blt/tested.

For now it's just the amp and PS modules.

Oh and a bass extender PCB kit and a (contact me) sub XO board -
http://www.diyaudio.com/forums/showthread.php?s=4ef609fe2783bc34fff37f9cb3c30c5f&threadid=69772

purpose tailored off my sim.


Hi ctviggen,

Crazy isn't it $100 - but I bulk buy components and my kids help with the Vgs and beta matching. they're learning enterprise and earning pocketmoney! Takes all the hassle of component sourcing out of DIY!

Cheers,
greg

[JoshK]

Well I had always been under the impression that lots of global negative feedback was a bad thing (lots of IM, higher harmonics, etc).  But the price is right that I will just have to order a pair and try it out for myself.  Turns out that the PSU for my UcD400s is about right for the GB150Ds, so I can try them out without much investment.

[amplifierguru]

The amp has very low conventional distortions typ 0.003% 2HD at 12.5W (a nice avg power level)  and <0.025% of 2HD and 3HD at 100W with the nasty high harmonics over -10 dB below.

Does this explain it's tremendous clarity, natural and tireless sonics?

You'll be surprised!

Cheers,
greg

[mgalusha]

Received your order - thanks! Your post highlights one great application for these compact amps - rebirthing a gutted old amp chassis utilizing the chassis, heatsinks and power supply. An excellent resource friendly economy.

I look forward to receiving and building these. I pulled out one of the chassis I'm using this evening and measured the output voltage of the PS. I have 64.5VDC on each rail, way too much so I'm going to have to procure some different transformers. I couldn't remember what it was and I thought it was in the mid 50's... Ah well, it will still be a fairly inexpensive project.

mike

[amplifierguru]

Hi mgalusha,

You could always purchase the larger amp, the GB300D, instead - it's 7.1" x 2" and 65v is right in the middle of it's 55V to 75V operating range.
Only $63 more delivered for the 2 kits substituted.

What's a traffo + hassle worth?

cheers,
greg

[ctviggen]

Which one of the power supply PCBs do you recommend for the smaller amp?  Will this sale be available as of next (not today) Friday?

[amplifierguru]

The GB150ABS power supply kit provides +/-14,100uF 63V plus a 6A discrete bridge , HF bypass C's,(front panel) LED support and bleed down resistors.
It's designed to mate with one GB150D as a monoblock utilising a typ 200VA 30-0-30 to 35-0-35 traffo.

Kits are US$80 for a pair (stereo).

The 3 per supply 4,700uF are rated 3.5A each and are close mounted in parallel for low supply Z. The PCB can be fitted with an edge connector for supply connections or hardwired.

Sale ends Mon 13th.

Cheers,
Greg

[mgalusha]

Greg,

Thanks for the thoughts on using the 300W board. I've made some measurements and I believe it will work just fine. I do have one question, what is the Zin of the amplifier?

Thanks,

Mike

[amplifierguru]

Hi Mike,

Both amps have Zin = 27K //1000pF so are an easy load.

At the heatsink you will need a flat accessible area of 7.1" x 1.2" for the GB300D and 4" x 1.2" for the 150W GB150D.

Hope that helps.

cheers,
Greg

[mgalusha]

Thanks Greg.

That's perfect. The heatsink in these chassis are about 8" wide and 6" high, so I don't see a problem.

[BradJudy]

Sounds like you've got a good plan going Mike.  I hope it turns out well.  Perhaps I'll even get to see/hear it.

[yo2tup]

i wish i had DIY skills  

[Gordy]

i wish i had DIY skills  

There are a lot of tutorials out there yo2tup, here's just the first one I found googling "soldering" ... http://www.epemag.wimborne.co.uk/solderfaq.htm

You could pick up an old receiver at the local flea market and take it apart for practice... Enjoy  

[JoshK]

never a better time to start learning than the present.  it really isn't as hard as it looks.  steep learning curve but once you start you can't stop.

[mgalusha]

... but once you start you can't stop.

Ain't that the truth.