Amp Camp Amp - ACA

[clayman0403]

Can I use BC550C as a replacement for ztx450?

[Zen Mod]

yup

[tyerkey]

Hi All,

I just finished my amp camp kit, and I have a few questions. Over the past few months, since my grandson destroyed my vintage stereo, I built a DIY system. Here is what I have; A Bottlehead Quickie preamp, 2 amp camp mono-blocks, and a set of home build speakers with Fostex FE126En drivers. My current music source is an iPod. All of these DIY components seem to be working properly. My problem is low volume. If I max out the iPod and preamp, it is almost loud enough for casual listening. There are a lot of variables in play here, so I will just list my questions.

1) Is 5 watts just not enough for these Fostex drivers?
2) The Fostex drivers are supposed to require 100 hours of break in time. Mine are on hour 2. Will they produce more volume as they break in?
3) What other changes should I expect as they break in?
4) Is it possible using the iPod as the music source through the headphone jack causes the low output? If so, will a 30pin iPod to rca adapter help?
5) Is it possible that I did something wrong with the amps that is causing low output? They check out fine per the build guide.

If you have any ideas or knowledge about these questions, please send them my way.

By the way, the sound that I am getting is really cool. If it weren't for pushing the preamp so hard, it would be crystal clear and balanced. If I can get the output to improve just a little, this system will be very nice.

Thanks!

[zany]

Hi tyerkey,
from a quick search on the net, Ipod out is approx 1.1V RMS output - into your bottlehead quickie (7.6db of gain) > amp camp (14 db gain) - probably not enough gain in that setup. Try connecting a dvd player that can play audio discs to the system (hopefully it puts out the standard 2 Volt RMS signal and see whether that is loud enough for you.

[billyk]

Originally Posted by Macfixer

That amp sure loves the gratefull dead

So I am not the only one to notice that my favorite shows sound so much better!!

[Loudthud]

Originally Posted by Samuel Jayaraj

One question - IIRC JLH had mentioned about lowering distortion by using a Darlington BJT for the lower output device. Any comments?

A bipolar Q1 would lower measured distortion, but the sound of the amp would change. Nelson likes to limit feedback to around 10dB. This keeps the distortion spectrum mostly 2nd and 3rd order. If you want to use biploars, use the JLH design, you might try reducing the gain with a resistor in series with the emitter of the PNP input transistor. Distortion of the ACA can be reduced with source degeneration to Q1 and selection of R1-R2/R3-R4 as I showed in posts 274 and 284 of this thread. A can't say that anybody would like the sound, but you could eliminate or reduce feedback.

I was noticing some inconsistancy in the numbers I was measuring. I discovered that the distortion changes significantly as you adjust the quiescent voltage pot. Below is a plot.

Trying different Q1s made a difference in the distortion vs frequency at 1W graphs. I also plotted the open loop gain. You can see that the smaller FETs have less gain but wider bandwidth. The smaller FETs don't change the low frequency distortion much. I think this is because the FETs are made of small cells in parallel, the bigger FETs just have more of them.

[Samuel Jayaraj]

Originally Posted by Loudthud

A bipolar Q1 would lower measured distortion, but the sound of the amp would change. Nelson likes to limit feedback to around 10dB. This keeps the distortion spectrum mostly 2nd and 3rd order. If you want to use biploars, use the JLH design, you might try reducing the gain with a resistor in series with the emitter of the PNP input transistor. Distortion of the ACA can be reduced with source degeneration to Q1 and selection of R1-R2/R3-R4 as I showed in posts 274 and 284 of this thread. A can't say that anybody would like the sound, but you could eliminate or reduce feedback.

I was noticing some inconsistancy in the numbers I was measuring. I discovered that the distortion changes significantly as you adjust the quiescent voltage pot. Below is a plot.

Trying different Q1s made a difference in the distortion vs frequency at 1W graphs. I also plotted the open loop gain. You can see that the smaller FETs have less gain but wider bandwidth. The smaller FETs don't change the low frequency distortion much. I think this is because the FETs are made of small cells in parallel, the bigger FETs just have more of them.

I made a mistake with that post. In the subsequent post I clarified that in the case of ACA, the darlington would be Q2, the CS. Q1 would still be a Mosfet as the amplifying device. This arrangement lowers distortion according to JLH.

[Loudthud]

I don't think JLH was speaking of an amp that works the way the ACA does.

In the ACA, Q2 is not a Constant current source, it looks more like a negative impedance load. Because of the way the Drain of Q1 is coupled to the Gate of Q2 via C2 and the load connection to the junction of R1 and R3, an increase in current by Q1 results in a reduction of current in Q2. Q3 regulates the voltage between the Source of Q2 and the Drain of Q1. If the current in R3 goes up, the current in R1 must go down. The voltage across Q2 goes up but the current goes down. That's a negative impedance.

In the selection of values for R1-4, Nelson has introduced a nonlinearity which gives the ACA it's pleasing sound. Replacing Q2 with a CCS would improve linearity but the quiescent current would need to be increased. There would still be some nonlinearity because Q1 is operated in a nonlinear region. Simply increasing Q1's current reduces distortion. The bridge configuration really cuts the distortion numbers due to increased current and so called even order harmonic "cancellation" (I think that term is misleading).

Experiment, have fun. I've built five of these amps, two stock, one with IRF120s and two with IRFP460s and other changes.

[Samuel Jayaraj]

Thanks Loudthud for your responses and insightful post above. So of all the versions which you have, have the 120s scored over the 240s?

[Jubelixi]

is it true that I can not use the Jordan JX92S speakers with this amplifier because the amplifier power is 5W?