Looking for a Low Wattage Amp Schematic.

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I have been stockpiling parts from dumped electrical stuff and I just picked up a transformer that I want to use to build a low power amp. I have a fist full of misc. transistors, some pairs and some singles, both MOSFETS and Bi-polars, and a bunch of caps and a ton of resistors and etc. enough to build a simple circuit integrated amp hopefuly to run a small sound system.

Tha transformer is a single secondary 20V AC that pumps out around 10 amps. So I figure that I could build something around 35 - 45 watts with that voltage and have a lot of current in reserve.

I would prefer not to build a class-A because of heat issues, but then again, I do like the JLH 20 Watt and the Nelson Pass designs. I just need something that will work with the limitations of the transformer.

Any thoughts,

Surf, Sun & Sound
 
A single 20V secondary will give a dc supply rail voltage of about 27V. This will provide a maximum power output of 10Wrms into 8ohm or 20Wrms into 4ohm. If you like the JLH Class-A circuit but do not want the heat, have a look at his Class-AB design on my website (link below), which uses a very similar topology but runs at a much lower quiescent current.

http://www.gmweb.btinternet.co.uk/jlhab.htm

[Edited by Geoff on 10-30-2001 at 05:23 AM]
 
JLH Class AB Amp

Geoff,

Thanks for the suggesting this amp. After looking around the web for the last several weeks, I have decided this amp will give the best performance for the transformer I have. I had looked at the Pass Class-A offerings and Rod Elloitt's DOZ amp and of the JLH Class-A 10 Watt, but with this transformer I would be able to only run about 5 watts and that is a bit low for any of the current speakers I have.

So here are a few questions before I start.

1. Could you give me an updated parts list as I am sure that some of the parts on the schematic are no longer available especially transistors and regulators?

2. I will be using the transformer mentioned in the above post, so I will be running about 25 volts +-, so what adjustments do I need to make to any of the resisitors or capacitors for that voltage? I will be using the PSU shown.

3. I am assuming that this amp will be sensitive enough to be driven directly by a CD player, correct? Will the lowered operating voltage change the sensitivity? I was planning to build in a passive pre-amp. If not I will need to drop the pre-amp supply rail to 16 volts because I was going to use Rod Elloitt's discrete pre-amp boards he has available.

4. Do you have a pcb layout available for this amp, or a component layout for bread boarding or point to point wiring? Since I am inexperienced at this, anything that will help me aviod a bunch of noise or feedback would be very helpful. Also advice on proper grounding or earthing would be very helpful to prevent a possible electrocution if there is a major failure.

I would love to do a point to point wiring job like is shown on the JLH 10 Watt Class-A amps that Rudy van Stratum built shown on Matthijs Tinholt’s site. Absolutely superb workmanship.

I know this is a rather long list, but I know that JLH amps are your pet projects and have more knowledge about them than anyone else I know of.

Thank you for your time,

Surf, Sun & Sound
 
SSS

What a lot of questions :). I'll do my best to answer them but we may need to continue this by email.

Updated parts:

Transistors: The BC212L, BC182L and 2N1711 are still current but you could also use the BC560 and BC550 (or a number of other alternatives if you have sourcing difficulties). The MJE521 and 2N3054 are still available but you could try a BD139 instead. The MJ481 and MJ491 are obsolete but can be replaced with 2N3055 and MJ2955, or other more modern output pairs from ONSemi or Toshiba (email me if you need a list).

You say you intend to use the PSU shown, but which one Fig 3 or Fig 10? If Fig 10, the stabalised power supply could usefully be replaced by an LM338K or current boosted LM317 circuit. There are suitable designs on my website. However, with your low transformer secondary voltage, I would be inclined to go with a simple capacitor/rectifier supply, as in Fig 3, (or possibly a capacitance multiplier) to maximise the supply rail voltage, and thus the power output, by avoiding the voltdrop across the regulator.

Several resistor values will need to be changed to suit whatever rail voltage you end up with and R1 to R4 will need to be adjusted to cater for your intended load impedance. I suggest you email me with this information and I'll determine the optimum values.

The input sensitivity will be fine for use with a CD player. All you will need is a 10k log pot on the input of the amp. If you do decide to use a pre-amp and want to supply it from the main power rail as shown in Fig 3, just replace the 24V zener with one having the voltage you require.

Sorry, I've no pcb or hardwiring layout for this amp but due to its simplicity hardwiring shouldn't be a problem. Just lay out the components roughly as shown on the schematic, with the exception of the feedback connection which should be taken from point 'X' on Fig 3 instead of the junction of R1/R2 (this will reduce distortion).

If the power supply is separated from the amp by more than a few inches, I would suggest adding a 100uF supply rail decoupling capacitor on the amp board.

For earthing, follow the guidelines on my website for earthing the JLH Class-A, the same principals apply. With regard to safety, there are no fuses shown on the schematic but I would recommend that you include at least a transformer primary fuse and preferably another in the supply rail.

Before laying out the amp and starting construction, I recommend buying/borrowing Doug Self's 'Audio Power Amplifier Design Handbook' and reading the relevant sections. The book contains a lot of useful information on the correct layout to minimise distortion, the correct earthing arrangements to minimise hum and various safety issues. Some of this information can also be found on Doug's website in the 'Distortion' and 'Earthing' articles.

Drop me a line if you need any further info.

Geoff
 
JLH Class-AB amp

Thanks a lot, Geoff for taking the time to reply to my many questions. I will continue this project by E-mail with you unless we want to use the forum so that others can follow progress and use for reference if they have an applicable situation. Your call.

I will use the capacitance multiplier PSU that you show on the Class-A site and Rod Elliott uses in Project 15.

I will get the complete circuit on paper including earthing and then I will let you take a look at it and give some constructive criticism.

Lisandro_P,

It would be easier for me to build the Rod Elliott Project 3A amp, but I do not have a transformer with dual secondaries. Since I live only a few kilometeres from Rod, it would have been a dream, but....

Anyway I want to build this amp, so there there is someone who has experience and give a honest opinion?!(yea, right!?)

Your call Geoff, the forum or E-mail, does not matter to me. I plan to post this project to the web, so you can link to your site.

Thank you,

Surf, Sun & Sound
 
Re: JLH Class-AB amp

surf said:
It would be easier for me to build the Rod Elliott Project 3A amp, but I do not have a transformer with dual secondaries. Since I live only a few kilometeres from Rod, it would have been a dream, but....[/B]

Tell me about it, i have to build my own transformer :D BTW a capacitance multiplier gives great results with circuits drawing almost constant current (like the JLH class A), but the benefits are not that good with an AB design. A simple capacitor filter would be much better with them.

Good luck with the amp!
 
Re: Re: JLH Class-AB amp

Lisandro_P said:
BTW a capacitance multiplier gives great results with circuits drawing almost constant current (like the JLH class A), but the benefits are not that good with an AB design. A simple capacitor filter would be much better with them.
The benefits aren't as noticeable vs. a Class A but there are still benefits. It's a simple and cheap circuit and if you have the room I'd recommend putting it in.
 
SSS

email or forum, I don't mind either but perhaps it would be better to keep the forum postings to aspects that would be of interest to others. Since my last post, it has occurred to me that with your low supply rail voltage the TIP2955/TIP3055 pair would be suitable for the output devices. It all depends whether you wish to use TO3 or flatpack.

Lisandro

Sorry, I haven't built the JLH AB so cannot comment on its sound quality but I have yet to find any of the JLH designs that didn't perform well. Even though I'm not too keen on his MOSFET amps, this is a personally preference and no reflection on the overall quality of the amps.

A capacitance multiplier will benefit both Class-A and Class-AB amps. The main difference is that in Class-A the improvement will be most noticable at low power levels whereas for Class-AB the benefit is at high output levels when the supply rail ripple noise is masked by the singal.

Geoff
 
Geoff said:
Sorry, I haven't built the JLH AB so cannot comment on its sound quality but I have yet to find any of the JLH designs that didn't perform well. Even though I'm not too keen on his MOSFET amps, this is a personally preference and no reflection on the overall quality of the amps.

I c... what about the JLH series of articles on preamps? Are you planning to publish them in your site and/or know where i could find them? I remember someone once mentioned a discrete preamp that performed great.

BTW, thanks a lot for the time and effort you spent on your site! :)
 
Heatsinks and transistors

Thanks, Geoff,

I will keep using this forum so that others can keep an eye on how we are progressing. 3 quick questions.

1. Heatsinks, how much and how many, for the PSU and Output?

2. I would prefer to use TO-3 pack transistors through out unless you feel there is absolutely no need to, of course I meant outputs, PSU & amp. The reasoning behind this is, that I want to buy quite a few(cheaper by 10's) and I will use the extras for a more powerful or Class-A amp later. I have some heftier transformers coming online. Also it would be great if we could use as few differents types as possible, again so I can take advantage of the quantity discounts. What do you think of that idea?

Here is a short list of the transistors that I can readily source -

BD 139 & BD 140(first pick for low power)

TIP 2955 & TIP 3055

BC 550 & BC 557

2N3055 & MJ2955(my first pick for high power)

3. How important is transistor matching, especially outputs? This would be easier if we could keep the transistors as similiar as possible.

I am in the middle of putting the final complete schematic together will try to post soon.

Thanks,

Surf, Sun & Sound
 
Lisandro

I hadn't planned to add any JLH pre-amps to my website, but in view of the interest shown by yourself and others I may have to have a rethink. The only problem is that there have been so many published designs it would be difficult to determine which to include. Perhaps I ought to drop the Hiraga, Sugden and other non-JLH articles from my site and just concentrate on JLH circuits.

SSS

Heatsink size will depend upon your final supply rail voltage, choice of output devices and on the design load impedance. Rough calculations indicate that for a 25V supply rail and 4ohm load you will need about 2 to 3degC/W for each output transistor and about the same for the Capacitance Multiplier pass transistor if you have a single power supply feeding two channels. I can firm up the sizing when the variables have been determined.

Go with your preferred choice of transistors. There is no matching to be done as the input stage is a single transistor (a differential input stage would benefit from matching) and it is nearly impossible to match a 2N3055 with an MJ2955 for the output due to the different device characteristics.

Geoff
 
Geoff said:
Lisandro

I hadn't planned to add any JLH pre-amps to my website, but in view of the interest shown by yourself and others I may have to have a rethink. The only problem is that there have been so many published designs it would be difficult to determine which to include. Perhaps I ought to drop the Hiraga, Sugden and other non-JLH articles from my site and just concentrate on JLH circuits.
(...)

Actually i found the Hiraga's designgs quite interesting too... i asked because i had no luck whatsoever trying to find a JLH preamp on the web :( Again, i'll be looking foward for it!
 
JLH discrete pre-amp

Hi, Malcolm here.
I have a paper copy of JLH's discrete pre-amp intended for his class-A design. Page 306 on, Wireless World July 69.
In the past, I have not had any problem ordering photocopies of articles like this via the Public Library.
If you are really desparate, I could scan a copy and send it to you.
Regards, Malcolm.
 
Re: JLH discrete pre-amp

Malcolm Jenkins said:
Hi, Malcolm here.
I have a paper copy of JLH's discrete pre-amp intended for his class-A design. Page 306 on, Wireless World July 69.
In the past, I have not had any problem ordering photocopies of articles like this via the Public Library.
If you are really desparate, I could scan a copy and send it to you.
Regards, Malcolm.

I'd love to have that if it's no problem to you! You can email me at Lisandro@Hotmail.com . Thanks in advance!
 
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