Baker HB7 Amp Modules

It started as the design for a small cased power amplifier I'd intended to market. I wanted something with strong visual appeal and retro looks, including backlit power meters. You know my views on amplifiers so it needed to be reasonably inexpensive and that's where I hit a problem. Add up the costs to build the things and factor in a reasonable return and, well not so viable as I first thought. Sure I could sell them for £1k but not when I believe a £250 Cambridge amplifier offers the same sonics.

This left me with a versatile power amp pcb design and so I decided to offer this to constructors looking to build their own amplifiers, active loudspeakers etc.
The boards are very small and include everything except the transformer - so good to go with minimal additional components.

It's based around a TDA7293 mosfet device which allows flexibility over power supply rails and output capability, and includes the option to drive the front end of the circuit via a separate supply is required.

It's a nice clean little 50w amplifier as supplied and well suited to DIY projects, or perhaps even fitting inside existing vintage gear where the owner wants to retain the classic looks.

The other spin-off from the original power amplifier project was variable output impedance. This partially mimics the drive characteristics of a typical valve amplifier and better suits certain vintage loudspeakers, and a few modern ones designed for valve amplifiers. This is now available as standalone box and simply sits in the loudspeaker cable line:

HB7 module:


1367446152

The aborted full power amp (prototype):


IMG_0125


IMG_0131 by trebor1966, on Flickr

OTX Emulator:


Emulator 2
 
The PCBs look an attractive proposition but the proposed anp I would not have found attractive.

A simple silver or black box with a source selector and a volume control on the front would have been more my style.

But those boards could be tucked away inside many an old speaker. How do do-it-yourselfers fare when it comes to fire regulations and household insurance?

You don't identify suitable transformers - catalogue numbers might be useful.

And how about some detailed DIY instructions for

1. a stereo amp in a standard catalogue box

2. a mono block.

Of course, as you know, at the moment I have a spare Quad 405 to get back on the road; so the HB7 modules would be of no immediate interest to me.

But later . . .
 
Hi Brian,

Yes the amp styling is very much the sort of thing I like, but many others don't. Another reason it was ditched!

The boards come with connection instructions and the site page gives the requirements for a transformer and heat sink requirements. Beyond that the case can be anything really.
Might be an idea to add a couple of recommended transformers and case though - thanks for the tip.

As for fire regs and insurance, there is no mains product supplied and the dc voltages are below 50v, so responsibility rests with the constructor. It's the same position as if you built and designed them yourself.

A fully built commercial amplifier is a different kettle of fish requiring stringent CE compliance. Though looking around at many of the commercial valve designs, few if any will really pass CE!
 
RobHolt said:
Might be an idea to add a couple of recommended transformers and case though - thanks for the tip.
Click to expand...

The data you send out with the amp could usefully be on the website with as many DIY tips as you have time for.

If those who have used them are prepared to submit details of their projects, and even (especially) pictures that would be even better.
 
If you could find a nice DIY case to make an amp pack to fix on the back of speakers that would be attractive for many DIYers I think. It would need a minimum of holes cut in the case and lends itself to the mono cards with their own rectifiers on each.
 
Tenson said:
If you could find a nice DIY case to make an amp pack . . .
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Great idea. There was a mail order company selling such a product a few years ago.

It can be an excellent solution, but remember you end up with two cables running to each speaker - signal and mains.

The box has to double as the heat sink, I guess. I wonder what might be found in the big selection of steel boxes used for electrical installations. Some of them are very well finished ,and they might be cheap compared to electronic project cases.

The stereo amp too has it's advantages - it's then located, in view or hidden, with the source, - an iPod, Airport Express or similar - and the addition of a dual gang pot in the box is a cheap and worthwhile.

Caution:

2 x HB7 £150
1 x transformer £25
Case and bits £25

Total £200

How much for a second hand Quad 303 or 306?

But the 303 or 306 may need a refurb.
 
Labarum said:
Total £200

How much for a second hand Quad 303 or 306?

But the 303 or 306 may need a refurb.
Click to expand...

This is a big issue Brian, and would be for anyone looking to produce amplifiers built here in uk in small quantities. But I'll have a think about the many fine ideas in the thread :)
 
RobHolt said:
This is a big issue Brian
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Sorry to be the bearer of bad news!

And then you have the problem of the price of a box from Richer Sounds.

Budget Amps with decent specs are cheap.

AV receivers have N x 100w amps, an on-board DAC and all the source switching you could need. They even come with DSP room correction routines. Not being subject to HiFi snobbery they are relatively cheap.

I do, however, have lingering doubts about such modern kit - the speakers are sparkled up to sound impressive, and the amps can fizz your ears to destruction in spite of what the distortion figures say. All that glisters is not gold.

But I have my spare 405. I continue to ponder - maybe a wooden plinth is the answer. I could then get under the amp a front facing volume control, source selector and mains switch. I could add a 3.5, jack input for iPod connection; and there would be room in the plinth for an Airport Express, Raspberry Pi or whatever.

(Have you seen how a community developed Squeezebox replacement is going?)

New lamps for old?
 
Brian what you suggest reminds me of the isolation plaform that was a DAC from many years ago. Was it Mission or Cambridge?

I think selling the HB7 amps as a module puts them in a different market from the amps you can buy ready made. DIY of course is not only about getting the best kit for the money, it is about having fun making stuff. If you compare with other amp modules around they are well priced and nicely made. Rob doesn't say much about it but they can also run on much higher voltage rails if desired and will push over 100watts.

I haven't followed anything about the Squeezebox, is something taking form to replace it?
 
I'm a lazy git by nature and would really need leading by the hand in selection of a basic case and transformer. I love the trannies available from RS which are fully encapsulated in a black resin? Absolutely silent unlike the ones still used by a cerain Salisbury-based manufacturer - cough - .....

The idea of a very plain remote power amp-pack able to be discretely placed close to the speakers via long but good, low capacitance interconnects really does appeal though :)


One other project you could look to Rob, is a cheap line driver/preamp. TEC do one available for around fifty notes - http://www.phonopreamps.com/tc754pp.html although I understand from someone who bought one recently that it might be a bit 'rough' by standards of its betters. I use an ancient but slightly updated old Amcron/Crown IC-150 preamp which despite its age, wiring, pots and switchgear and coupling caps plus output carbon resistors (to give a 600 ohm output impedance), sounds incredibly transparent to the input signal (accepting mine now has OPA134's and Nichicon 1uF input caps replacing the axial tants and previous op-amps - not LM301's in mine). I'm just saying that a simple but clean low current power supply feeding a pair of sensible op-amps may be all you need, plus switching for inputs of course...

S'cuse the convoluted rainy-Sunday musings. Always a pleasure to talk to you online :)
 
Those TEC cases look suspiciously like the same boxes Beresford uses for his older DACs!
 
Dave, yes I'm a fan of the encapsulated transformers which often have lower mechanical noise. I'll pop a photo up later showing the inside of the amp above with such a transformer..... but I've got roast pork followed by jam roll and custard to get through first :)
 
RobHolt said:

PC200723
Click to expand...

Just looking again at the quality of that case (and of the wiring looms.) Very nice.

We have noted already how difficult it is for a small supplier to market a small amp at a price that will sell, given the price of competent 50w amps from the likes of Richer Sounds, but I wonder how the economics work out for bigger amps?

If a Quad 909 (or whatever it's called at the moment) sells for about £1000 it must surely be possible to undercut that by a serious margin using fine quality components.

I once worked in a Physics research lab where most of the custom electronics was built using a 19" rack kit system - big sturdy box, hefty transformer, heat sinks current dumping boards in the 520/606/909 blood line. They need no setting up and cannot go out of alignment.

And, heresy of heresies, what happens if you marry current dumping boards to a switching power supply?

Do the economics stack up?
 

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