Open to suggestions!

This is my first post so please be kind!

I am looking for a preamp project to build to replace my Hart Kit JLH 30W Mosfet amp and preamp.
I have limited equipment available and limited (and reducing!) technical knowledge of amp design.
My personal preference is that amps are 'colourless', they add and take away nothing. Minimum distortion and background noise.
My current setup sounds very good but has too few inputs, nothing digital and certainly no remote. I want something up to date.
Provided I make a sucess of the pre, I will then want to move onto the main amp, seperately boxed.

If anybody has any suggestions of what I could build, bearing in mind my limitations, I would be grateful!

pnb
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
Hi pnb
I'm surprised others haven't replied before to what is a fair question and common situation.
Briefly, there are dozens of DIY level basic preamp projects ongoing in various forums here when you count in the commercial and cross-forum threads. Having PCBs with complete details for your construction is not normally on offer, however, unless you have joined in a construction thread.

If you want to cut to the chase, there are a couple of very popular websites, Rod Elliot's ESP particularly, which offer a wide range of projects, details and PCBs at fair prices. This one has a forum, good theoretical discussions, tutorials, notes and comments too.

However, it appears that you want all the remote, digital bells and whistles of current commercial equipment which, though discussed and occasionally offered as a project, is getting too complex and expensive for most DIY interest. This owes to the fact that they are microprocessor based devices and usually now require SMD assembly. However, some Asian manufacturers market a convenient range of pre-assembled, working electronics bundles containing all the remote control and preamp features. These are amazingly cheap and effective considering the price of commercial equipment containing little better. Epay stores are one such source with UK and China based outlets often referred to here.

DIY guys usually shun this type product for the reasons you give already, suspecting that 'colourless' qualities will be somehow compromised. That could be so but IMHO, most compromise to sound below 0.01% RMS is going to be psychological or actually due to peripheral issues like poor interface design and implementation. Modern audio electronic devices are surprisingly good and convincing when you forget the hype and realise that millions now enjoy MP3 quality from their PCs hooked into their much higher quality audio systems. Crazy, when you think about it but just as when audio cassettes ruled the domestic Hi-Fi scene, ignorance is bliss!

The nett outcome is that few DIY projects include such convenience features and take the purist approach with minimal controls and features. Have a close look at at Bonsai's recent X-Altra preamp thread in this forum.

Here in Oz, there are complete kits for preamps, amps, DACs and all manner of peripheral equipment for any average constructor to enjoy but similar restrictions still apply to digital control features. With the exception of remote volume controls with simple pre-programmed DIL micros, they are too difficult to build and expensive to source for small volume purchasers and average DIY constructors.

There are some fair alternative approaches if you are mainly interested in convenient, multi featured control systems and I would not discount good, used commercial products.
As a functional basis, these can often be simply "upgraded" in the true DIY style.

Hope you get a chance to check out what's on offer out there.
 
Ian,
Thanks for your detailed reply, I will start to check out some of your suggestions.
I have allready identified a DAC on ebay that could offer the digital inputs and if I have some sucess with that I will then search out a micro controller solution.
I think, as you say, these will be the trickiest parts to produce, the analogue amps being a little more available.

pnb
 
modern stuff

I got a magazine full of modern kit demonstrator by ordering from Mouser.com. The magazine is element14.com, I'm imagine they have online stuff.They have an online TV show webcast if you're up to that. Looking forward to remote on/off and remote volume control, I've been salting away used VCR's at $5 for the box and $7 with accompaning remote. They have remote on/off, remote tracking, which involves a motor, and remote volume which might be primitive and might be integrated into the microprocessor in later units. No way would I use their sound parts, but the relays and tracking motor might be useful for turning a phyical pot.
 
Took a picture of my amps insides and added some annotation
pnb
 

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Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
Thanks for sharing, pnb. It seems a good build of a reliable design, as I knew of it.
With the main caps in good condition, it might be just great for the new preamp to work into.

Unless I'm mistaken, it appears that the front panel is removable so perhaps you could make a new one eliminating the tone controls, disabling the preamp and giving space for a display panel and remote sensor. You may then even be able to replace the volume control with a rotary encoder. A bit radical, but it would save the 2-box arrangement and duplicated controls...Thought?
 
I replaced the caps a couple of years ago so they should be ok for a bit.
Its only shortcomings, ignoring extra bell and whistles, are that PSU is too simple and there is audible hum and that the speakers are not 'protected' from turn and off thumps and burps.

Yes that may be possible to replace the preamp section, the outer extrusion hides the upturned steel chassis base so I could easily remodel the steel work and then cover it up with a new ali fascia.
I will try and scan the schematics and supporting text, they make interesting reading 30 years on!
IMHO JLH was an outstanding designer and a pioneer of the use of MosFets particularly in power Amps.

pnb
 
I am looking for a preamp project to build to replace my Hart Kit JLH 30W Mosfet amp and preamp.
using Hitachi's renowned Lateral mosFETs.

Yeh, keep the PSU and RIAA and Output boards.
remove the volume/tone control stage and replace it with some buffered inputs and a +6db/+12db gain stage for any low level signals you might come across.
Consider adding a +5V PSU and a LED/LDR volume control.
 
Lovely looking build.

If you are happy with the sound then do please hang on to what you like. How about just building the pre-amp that you need - with all the remotes etc. - and have it go into one of the line level inputs. That way you will only have to get up for the turntable or whatever else you leave connected. You could either do this yourself or pick a kit or choose something where you can modify the signal path components.
 
Thanks CT I am considering that but what I need to find is a pre-amp solution that is a kit as I do not have the expertise or equipment to create a solution.
Any suggestions welcome!

A friend has kindly scanned the JLH articles that detail the circuits I currently use. I hope it is of interest.
I can't believe they are 30 years old !!

The first is the overview with a complete schematic, except for the power amp. This is because JLH first designed it with epitaxal output transistors but then later in December that year converted it to use Mosfets.
The modification was slight but effective.

pnb
 

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Mooly

Administrator
Paid Member
2007-09-15 8:14 am
It seems a shame to "hack" such a finished article about... not that it can't be done but be under no illusions about the work involved.
That front PCB could be redesigned adding an ALPs motorised pot, the inputs could be switched electronically too.

This is an excellent project using a PIC... the best by far I have ever seen.
FPRC5RX - DIY learning IR decoder

I used a similar idea here to control MOSFET switches rather than relays,
http://www.diyaudio.com/forums/soli...fet-amplifier-designed-music.html#post1452488

It's a huge amount of work to integrate it all though.
 
Thanks Mooly for your thoughts. I read your links with interest
I recognise the work involved and as I do like what my current setup provides I think I could will concentrate on 'improving' it.
A cleaner PSU to remove the audible hum
Some kind of circuit to cut off the switch off burps.
And an enhanced preamp, in a seperate box..

However I have found that it is still possible to buy JLHs 80W Amp and PSU pcbs, and that is proving tempting!

pnb