Amp Camp Amp - ACA

[ta240]

HF performance will suffer

btw. input resistor (R11) is directly in correlation with other feedback net resistor (R12) thus defining both input impedance and gain

Thanks for the quick reply. That is what I was afraid of.

It was suggested elsewhere that I add a B1 buffer after the preamp. It doesn't look like that would be too expensive of a solution.

 

[vdi_nenna]

Is there an easy way to raise the input impedance without messing up anything else on the amp? Just raising R11 seems too...

Just curious, what's the output impedance of the tube preamp?

 

[ta240]

Just curious, what's the output impedance of the tube preamp?

It is showing a maximum of 68k

 

[Zen Mod]

that's not preamp

more like reffub

 

[hlawka]

ACA kits sounded great after construction completed; but left on overnight one was cold and non-functional next day (LED still on, though); other was still OK. Where to start trouble-shooting?
-Thx

 

[vdi_nenna]

Check the voltage on drain of Q1. Also Q2.
Check incoming voltage.

 

[nzlowie]

Hi guys
Have a question around setting bias of this amp. Does it make any difference if we attach a load to the output? Someone told me I needed to but never read it here anywhere.....

4 Channels on my 420mm x 275mm heatsink runs way too hot, can't touch it!!! Just wanted to make sure my bias setting method was correct, no load and 10v on the Q1 drain.

Is it correct that these run at about 26 watts load or no load? So that means my setup would be around 100 watts at idle, hot.....

Thanks

 

[Zen Mod]

without Iq mod (one resistor 2K2) , Iq is app. 1.1A

that is around 21W of heat per channel

setting is just 10V in front of output cap

while setting - input grounded , load irrelevant

 

[Dennis Hui]

Hi Nzlowie,

What rail are you using now? (I noticed you mentioning the desire to try 24V)

I really like the look of your case but I suspect the heatsink is not getting adequate
ventilation for cooling.

Dennis

 

[ChrisG139]

24 volts

Hi Everyone
I would like to try 24 volt supplies but I'm not sure if this will cause damage if I just plug the supplies in.
Thanks (and pardon my naivity!)
Chris

 

[Zen Mod]

just plug

then set (trimpot) inner side of output cap 5V higher

 

[ChrisG139]

24v

Hi
Once again my apologies- could you please point me in the right direction?
Thanks
Chris

 

[Zen Mod]

to repeat my reply on your question :

just plug them in , then repeat setting procedure , this time setting inner side of output cap 5V higher than for 19V supply

that would be 15V

take care of hsinks temperature , to keep them in 50C bay

 

[ChrisG139]

Hi Zen Mod
I bought the amp second hand so didn't build it.Therefore I'm not sure how to set the inner side of the output cap
Any help will be greatly appreciated.
Thanks
Chris

 

[Zen Mod]

all help you need is here: http://www.firstwatt.com/pdf/art_amp_camp_1.pdf

 

[zekeblue]

Hi Zen Mod
I bought the amp second hand so didn't build it.Therefore I'm not sure how to set the inner side of the output cap
Any help will be greatly appreciated.
Thanks
Chris

Chris,

You need the pictorial build guide. Here's a screen shot of the adjustment. Multimeter to Q1 and ground. Set the pot (blue plastic square with white screw adjustment) to 15v (or whatever your target is). Keep cover on as it will be when finished. With amp on, wait half hour and adjust again.

 

[ChrisG139]

Thanks Folks
That's brilliant!!
Will give it a go at the weekend.
Regards
Chris

 

[kpsthakur]

How to replace of 2SK170 by J111

Nelson Pass mentioned Fairchild J111 being a possible substitute for 2sk170 for F5. Only $0.16 each, 73,000 parts in stock at Mouser. It seems like it would work very well in the ACA

I have exhausted my 2SK170's I got from US and am actively looking for replacement JFET's. J111 as per these comments looks like a great candidate, got them super cheap from RSDelivers.in however it doesn't look like it's an apple to apple replacement for 2SK (.. tried replacing them in my working ACA channel..including considering the leads are different for J111), but did not work...guys, need some direction/headstart to make this possible.

 

[Zen Mod]

J309

when you have it , buzz and I'll tell which resistor to change

 

[kpsthakur]

J309

when you have it , buzz and I'll tell which resistor to change

Thanks ZM

J309's are available and not too costly, there are 3 options ON Semi/NXP/Fairchild .... all smd [emoji31]