Nick G modified Lentek Head Amp

[Arkless Electronics]

Probably the biggest improvement one could make to a standard Lentek would be to replace the emitter capacitors with film types such as polypropylene. In Nicks mods he increases them from the standard 10uF ( I haven't been able to read the lettering on these caps so I don't know 100% that these are originally 10uF but I would put a lot of money on it that I'm right!) with 47uF ones , as I would also, and the issue here is a purely physical one... together, the 4 caps would be bigger than the whole head amp!! Re-boxing it in a much larger case would be required.

Edit: I may have lost a lot of money on that bet as well!! In close up photos of the original innards all the tantalum caps look identical (it's a pity the photos at the beginning of the thread are gone!) and as most are marked as 10uF in Alan's photos (which have some errors BTW) I have assumed all the identical looking originals are 10uF....
HOWEVER.... If this is correct then the original version was -3dB @ 50Hz!! The 47uF ones are suitable...
As with most cap types, tants get bigger as they go up in capacitance and/or voltage rating. It is possible that the originals were indeed 47uF but at probably 6V rating whilst the identical looking ones at 10uF are rated for 10 or 16 Volts... It would be interesting if anyone can confirm. Neither is even slightly critical and the more capacitance the better in both cases. All other values I am certain about as clear enough views of the parts were available.

[Marco]

The twin battery idea I'm talking about is 100% different to that which you are no doubt talking about and involves a complete redesign of the circuitry whilst keeping the basic topology. It would likely only be practical in a new Arkless version. However, there are reasons why twin batteries as you are probably talking about would also be an improvement yes.... if they will fit! A new on/off switch would have to be fitted as well.

I hear ya, and cool. We'll cross that bridge when we come to it. I'm probably more focussed on the Celestion mods at the moment, but further surgery on the Lentek will come soon after that

Marco.

[Arkless Electronics]

Marco, do you find the output rather healthy from the Lentek? Simulation predicts 28dB gain....

[Marco]

Yes, very healthy - and I like that. We've been here before, in terms of a healthy gain contributing to a 'punchy' sound, as opposed to the opposite, when it's 'weedy'

That effect, however, will be very cartridge and/or partnering phono stage dependent.

Marco.

[Arkless Electronics]

Yes, very healthy - and I like that. We've been here before, in terms of a healthy gain contributing to a 'punchy' sound, as opposed to the opposite, when it's 'weedy'

That effect, however, will be very cartridge and/or partnering phono stage dependent.

Marco.

Hmmm... well as you know this is one of the (many! ) things on which we disagree.. my question was purely to see if the output seems as healthy in practice as computer simulation predicted and I would have been amazed if it wasn't. Sims predict THD of 0.002% BTW. No flies on that!

It could be made much less hissy if battery life was not an issue

[Marco]

Hehehe... Hiss has always been a factor with the Lentek, so anything which notably reduces that, or nullifies it, would be great

The key for me with the Lentek is to maintain its rather 'feisty' and upfront way of music-making [not sucking the life out of it], whilst injecting it with a soupçon more resolution and refinement. That's certainly the goal I have with mine.

Marco.

[Arkless Electronics]

Hehehe... Hiss has always been a factor with the Lentek, so anything which notably reduces that, or nullifies it, would be great

The key for me with the Lentek is to maintain its rather 'feisty' and upfront way of music-making [not sucking the life out of it], whilst injecting it with a soupçon more resolution and refinement. That's certainly the goal I have with mine.

Marco.

Compromises as ever in engineering.... No hiss is impossible for a start! Reducing it means reduced battery life....

[Marco]

Yup, sure. Hence why the twin battery idea is of interest.

Marco.

[Light Dependant Resistor]

Yes but as usual, serious circuit analysis missing.

The circuit suggests it is a Class C device with no DC bias, rather just being turned on by
the cartridge signal. It would be nice to see it instead as a common base design -which it hints at, where voltage
increases, emitter then being input and collector the output. Better though as common base with a dual rail supply.
Also way too many RC time constants. Capacitors have their place in power supplies not in audio circuits
where they can be if carefully designed,can be totally avoided.

Fets and mosfets ( and variants like implementing cascodes ) being a much better choice as amplifying devices, for audio circuits.

[Arkless Electronics]

Yes but as usual, serious circuit analysis missing.

The circuit suggests it is a Class C device with no DC bias, rather just being turned on by
the cartridge signal. It would be nice to see it instead as a common base design -which it hints at, where voltage
increases, emitter then being input and collector the output. Better though as common base with a dual rail supply.
Also way too many RC time constants. Capacitors have their place in power supplies not in audio circuits
where they can be if carefully designed,can be totally avoided.

Fets and mosfets ( and variants like implementing cascodes ) being a much better choice as amplifying devices, for audio circuits.

Oh dear..... It's not class C, it's class A. The "cartridge signal" couldn't turn anything on or bias anything....

Common base would work equally well yes, and many variants of the Lentek circuit and similar common base ones are extant. Common base would make it virtually impossible to change cartridge loading though, and the gain would vary considerably with cartridge source resistance. I've not simulated it but a glance at the circuitry says to me that the standard Lentek input loading is roughly 90 Ohms.

Dual rail supplies are what I was hinting at to Marco earlier but getting the full benefits are rather more awkward than at first glance in this topology...
Whilst there are all sorts of obvious compromises, I can see exactly why, under the circumstances of it having to be small, powered by just a PP3 for a good long while and relatively cheap, they choose the blend of compromises that they did. Poor noise performance and needing electrolytics or tantalums in sensitive areas are the most salient down sides of the compromises.

For best results this circuit would also need the transistors to be not only selected for low noise (partial get round by choosing known very low noise devices and taking pot luck), but also carefully matched for gain and Vbe. It can give an output offset voltage of several millivolts, which could upset some following MM stages if DC coupled. Matching would help hugely with this, as well as overall SQ, but the devices should be thermally bonded for good thermal tracking.

Cascoding would be irrelevant to this topology and just be noisier and twice as complicated etc.

Good luck with building a mosfet head amp BTW