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Old 10th November 2010, 03:48 PM   #1
pnb is offline pnb  United Kingdom
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Default 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
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Old 15th November 2010, 02:39 AM   #2
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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.
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Old 15th November 2010, 09:52 AM   #3
pnb is offline pnb  United Kingdom
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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
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Old 15th November 2010, 03:22 PM   #4
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Default 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.
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Old 15th November 2010, 06:47 PM   #5
pnb is offline pnb  United Kingdom
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Thanks Indianajo, I will check out your reccomendations and will post again if I find a source of something that meets my needs.

pnb
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Old 15th November 2010, 09:04 PM   #6
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Hi PNB,

Do you by any chance have pics || schematics of your current setup ? You say you are happy with it's sound, maybe it's an idea to mod rather then to buy new ? As said by Ian, there are lots of modules available to do extra inputs or motorized volume knobs ...
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Old 16th November 2010, 09:16 AM   #7
pnb is offline pnb  United Kingdom
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I haven't any photos of my current amp but I will try and take some soon.
I have the original schematics from John Linsley Hood's articles along with the Hart Kit details. If I get a chance I will scan and post them.

pnb
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Old 16th November 2010, 06:32 PM   #8
pnb is offline pnb  United Kingdom
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Took a picture of my amps insides and added some annotation
pnb
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File Type: jpg Amp PCB.jpg (595.2 KB, 86 views)
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Old 19th November 2010, 05:18 AM   #9
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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?
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Old 19th November 2010, 09:51 AM   #10
pnb is offline pnb  United Kingdom
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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
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