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udderslate 6th February 2009 10:45 AM

new builder , guidance requested .

I've been lurking for about 2 years , drooling , reading , wishing .
I've been an elec tech for 20 years , but last 10 a programmer .
I sure miss flicking solder from my iron onto the floor !

So , I want to build some amps.
I'll start with dorm room PC sound for my two boys .
Likely for the small sat speakers , I'll go lm1875 or lm3886 ,
with active hi/lo pass set at 150hz .
or maybe some discrete amps ( dxII for starters )

I have a +- 47VDC (unloaded ) Pwr Supply from a 55wX2 rcvr .
Id like to use it for a sub amp , with a 4ohm 12in woofer . ( EHQS12 )
I'll take the power I get from it .
So , a simpler design works for this .

I have in mind protoboard , a real mans constructions..says loved Carlos . I have made PC boards before , should I do that ?

The DXAmp seems not happy with > +- 35v , so my choices are ostrippers FrugalAmp FA-1 , or Rod Elliots P68 .
Only because both are shown with voltages near my +-47v ps .
If the DXII can run with my +- 47 volts please tell me !

So ,Am I crazy ???
should I build these for my first amps , wired point to point ?
or should I only use PB boards ?
or should I build a discrete headAmp first ( rod Elliot P113 )
FYI: I don't have a scope , just a DVM and a PC .

I'm not looking for easy .
I want to learn , smell smoke , and hear satisfaction .

Could I get opinions about this , so I make the right start .

and yes , I know its cheaper to buy an ebay amp but I want to
make my own for the experience .

Ultimately , I do want to try all of these for my home studio .



h_a 6th February 2009 01:51 PM

Hi Wayne,

well, you could go with a small headamp for starters. has a large selection of interesting headamps and sells also pcbs which makes the start easier.

Building more complicated amps point-to-point is not exactly for beginners, on the contrary I would only recommend it for the experienced ;) Lots of problems can plague p2p, noise pickup, poor reliability, hard to repair, some amps may even start to oscillate. The increased distortion is maybe the smaller problem. Also testing can be hard and dangerous.

+-47V is already quite much to start, maybe you want to get a new transformer after you've decided what to build? Don't forget building amps is usually not cheap, so you could as well spend some additional bucks for a suitable transformer.

Anyway, enjoy your new hobby - and take care when testing!

Have fun, Hannes

ostripper 6th February 2009 02:18 PM


By udderslate - The DXAmp seems not happy with > +- 35v , so my choices are ostrippers FrugalAmp FA-1 , or Rod Elliots P68 .
Any of these amps can be ported to run at different voltages..
All I do is a quick simulation at the different voltages,
usually one just has to change resistors in the input/CCS
and VAS to get reasonable currents and then go to it.

Running the DX1 would be a little harder with the BD-XX
trannies , they can't take the higher voltage , but with a MJE340
,no problem. The DX 2 should run at 47V, but without changing
resistors ,everything would be "off".. (quasi has a resistor chart
to run his nmos amps from 45-70V.. cool)

You forgot about the Quasi amps, they are MOSFET ,but simple
as well.. The FA1, without changing anything , has been
run with any salvaged PS between 45 and 70V ,but it is always
preferable to optimize the resistor values/device currents
for long term reliability.

PCB's are always preferable to protoboards,
as the layout can add capacitance
at the wrong place making the amp prone to oscillate.

c2cthomas 6th February 2009 02:38 PM

Hi Wayne - and Welcome to diyAudio!

I had a 20 year diversion away from this little hobby as well and needed to do a bit of "updating - refreshing" when I got back into things. Seeing that you have been lurking the forums for a couple of years you have some projects in mind and some good advice from a couple of good constructors - I'll leave that up to you kids.

Keep up posted on your projects and progress! :cheers:

udderslate 6th February 2009 08:08 PM

Thanks everyone for the help . All points taken .
h_a, I'm not new p2p , just amps, so on that I AM a beginner .
Advice taken .
That said a dumb little PWM motor board was kinda a pain.
It did work though .
and i know , I can't build as cheap as can buy...but thats boring !

Os , I snatched your , I'll have a go at the sims .
( thanks for considering that I might be able to )
I did sim the p70 for a learning exersize,
just to get the hand of LtSpice .
a full amp will be the next step .
I'm sold..its PC boards for me .
I expect p2p will cause me trouble I don't need .
still...I'll give p2p a try for the p70 headamp..

I had a look at Quasi...hmmm.. I've been there before ,
why did I pass that up ( fear of mos ? ) ..what a great site .

c2cthomas: thanks for the welcome .
I do a bit of fixing here and there( local school audio gear )
and I did troubleshoot and fix my blown CS400 .
but , alas I haven't built anything from scratch for years . like 10 .


udderslate 8th February 2009 02:57 AM

I simmed your amp at posted +-60 , and at my PS of +-45v .
I wanna get the details don't answer more than yes/no,
or keep digging...or RTFM ! ha ha .

I noted the current ( at +-60v) of the input Re, and VAS Re .
At +-45 V , should I simply adjust the Re's to have same current,
as at +-60V ...and then maybe tweak the NFB to have same gain ?

By the way...what did you intend to be the max input V ( 1,5 ? )
I saw clipping after that .
I'm having fun ..thanks .


ostripper 8th February 2009 03:40 AM


I noted the current ( at +-60v) of the input Re, and VAS Re .should I simply adjust the Re's to have same current,
as at +-60V ...and then maybe tweak the NFB to have same gain ?
I won't tell you the values , just get the CCS current (Q5/R7)
3.5 - 4mA and 6- 8 mA for the VAS (Q6/8).

FA1 is problebly the most common bipolar amplifier (Doug self's
"blameless") with the least possibility for error.

And "yes":D , NFB anywhere from 22 -33 K keeps things "strait".

You could also build a "bootstrap" FA1, just tack 2- 3K3 resistors
between R13 and V- (get rid of Q8) and put a 100uF from
the midpoint to the output..(FA1 "B".asc) in the frugal directory.
Just another option for you to consider :) .

udderslate 8th February 2009 04:12 AM

Thanks, I had it it right then .
Funny, or lucky , at +-45 those values are 3.67ma and 8ma .
Close enough , I'll set it on the real build .

I've was just looking at the bootstrap on the P3a .
When I build this I'll try it as is , and with the bootstrap .
Um..whats the difference ? the P3a mentioned it help thumps .
anything else , besides simpler ?

Bought some HydrogenPeroxide , need Muriatic acid and boards .
Parts order soon .

I wish I had a flock of neighbor kids like you do ...
I gotta but heatsinks/ dollar .
done for the day now Thanks .

ostripper 8th February 2009 05:57 AM


Um..whats the difference ? the P3a mentioned it help thumps
SOME say bootstrap sounds better. big personal preference
issue with many amp people ;) . either way, fa1 has no thump
anyway because of CCS..

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