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PostPosted: Wed Mar 06, 2013 8:00 pm 
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Location: Malvern
A new place to put this stuff so I no longer bury the 'hifi for sale' with my meanderings.

My next project following the Quad modifications will be a bit of design work. Either a no compromise headphone amp, or a no compromise phono pre-amp. If there is any preference, what would you like to see? Obviously I'll be posting progress here for your delectation / bewilderment / insomnia cure.


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PostPosted: Wed Mar 06, 2013 8:32 pm 
King of the Skip Monkeys
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Location: Moomin Valley
headphone amp! portable!


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PostPosted: Wed Mar 06, 2013 8:50 pm 
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Portable... damn. To give decent rail voltages (and hence dynamic range) a DC-DC conversion is required... Challenging, though I seem to recall using split rail step up from single rail in a previous job in a design for an emergency comms system.


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PostPosted: Wed Mar 06, 2013 11:36 pm 
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8) phono pre amp...must be valve...obviously.....and adjustable pre/post so i can use it with my bass :wink:


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PostPosted: Thu Mar 07, 2013 1:00 am 
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Freak ;)


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PostPosted: Sat Mar 09, 2013 12:46 am 
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Anyway, todays Quad mods.

The 405 was produced with one eye on driving their electrostatic speakers with their certain requirements, and has voltage and current limiters fitted in order to protect said speakers. Both of these can impact dynamics as they impose non-linearities (in a pretty non-linear way in this case!) on signals with extreme transients or extreme dynamics. Therefore, it makes sense to remove these limitations if you are using this amp with conventional speakers. You can see the space on the circuit boards where the current limiters were, and the sockets for the voltage limiter resistors now empty.

Image

Haven't had a chance to listen yet, but whilst researching the V & I limiters found some pages by some chap called Keith Snook who is big into Quad stuff and has done a lot of work on them, and confirmed that the limiters have a negative impact. He has a lot of damn good ideas, so I'll take a look at his work and look at applying some of it to my amps.

1) The input op-amp (the voltage gain stage) by it's design is noisier than it could be due to it being of inverting design to maintain overall non-inversion. As this doesn't really matter a damn, it can be reconfigured to reduce noise.

2) The discrete class A stage current source can be simply improved easily.

3) The feedback circuit can be improved easily.

4) I am going to have the aluminium case parts anodised 3DV! (Just kidding, gunmetal grey like later Quads)

Anyway, more adventures tomorrow!


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PostPosted: Sat Mar 09, 2013 12:53 am 
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legrandefromage wrote:
headphone amp! portable!


Was talking to a mate down the pub, and we think it has to be Class A as you can have Class A for headphones without the huge heat load when you need speaker driving power. However, I'm fairly sure no-one will want to be changing their batteries every 10 mins, so for portable, that's out!

What is the reason behind portable headphone amps? Is the amp in iPods etc bad? What are the requirements? I can surmise that basically they'd be size (small), weight (light), battery life (damn!). More?


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PostPosted: Sat Mar 09, 2013 9:27 am 
King of the Skip Monkeys
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The EU has said that we're all going deaf due to too loud iPods and such like. So the volume output is limited by quitea lot.
I used an older set of late 1980s headphones and they were much louder than my current ones too so maybe there is some limiting going on there too.

The audio from the bottom of an iPod is much better than the headphone socket and is fixed.


As an aside I have a dead Cambridge Audio phono stage that could do with reviving.


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PostPosted: Sat Mar 09, 2013 9:48 am 
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Srnd it over, I'll sort it.


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PostPosted: Sun Mar 10, 2013 9:52 am 
retrobike rider
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Can anyone offer some advice on this valve amp?

The last time I studied valve theory was back in 1976 and that was to do with transmitters!

Anyway, this is a Chinese made amp that I bought from the bay a few years ago. It has been doing good service for my son on his computer until it went POP!.

I have not dug around in there with a meter yet, but it is obvious we have a cooked carbon (220k) resistor and a boiled capacitor too. Also, the valve itself is looking pretty sad.

So, before I get busy, or not, I have to answer the question did the valve take the components out or vice versa?

Any hints, tips and advice gratefully received.

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