Nick G modified Lentek Head Amp

[Marco]

Hi Paul,

I have to side with Jez on this.

The Lentek circuit is simple and elegant in its concept.

So do you rate the design?

The resistors used in Alan’s photos are thick film and metal oxide types which were available from RS components back when the Lentek was manufactured.

Ok, so when modding the unit do you recommend that these resistors are retained? I'm only saying that if you think they are in some way key to how it sounds, or if they'd be better replaced with something better from currently produced options?

Marco.

[Marco]

Some respect due to Lentek over their choice of transistors (of those available then).

Interesting, Jez, so what are they exactly and why is respect due? Are these transistors still available now, or if not what would you suggest betters them?

The transistors were actually the last thing I was thinking of changing anyway, as I suspected that they were a big part of why the unit sounds like it does.

Tis' a pity that for reasons of battery life, size and price the collector current is so low as to make it so hissy...

So could that issue be successfully addressed?

Marco.

[Marco]

Incidentally, for anyone considering building a Lentek head amp for themselves, the 2SA1085 and 2SC2547 with a source impedance of 100R both have a noise factor (NF) of around 0.5 dB and they are effectively operating in parallel for noise purposes, so the averaged noise contribution would be less than 0.5 dB without a cartridge connected. The noise level would rise slightly with a low impedance cartridge connected but would still be acceptable. These transistors are not current production items but they can still be sourced from surplus outlets.

So, just to clarify, for us numpties, the 2SA1085 and 2SC2547, of which you speak, are the transistors used in the Lentek?

Do any of you guys have some spare ones, then? Would be useful to know in case any currently being used go tits up. One other thing I'd like to do is house the whole thing in a non-magnetic case. The stock one is ferrous and creates a significant magnetic field around the components, which in my experience is sonically suboptimal.

Marco.

[struth]

think they are available here

[Arkless Electronics]

Interesting, Jez, so what are they exactly and why is respect due? Are these transistors still available now, or if not what would you suggest betters them?

The transistors were actually the last thing I was thinking of changing anyway, as I suspected that they were a big part of why the unit sounds like it does.



So could that issue be successfully addressed?

Marco.

They are no doubt available still with a bit of a search. To expound on my earlier post, transistors come in opposite polarities (and all audio EE's wish valves did!) and ideally in a push pull circuit we want them to be perfect mirror images of each other. This is normally done by using two transistors of the same series or family, which are intended to be complementary to each other. In the case of the Lentek they have gone above and beyond the call of duty and experimented with all sorts of transistors , but only those with at least some pretence of not hissing, to find ones that are more perfectly mirror images of each other than could be expected from choosing them from even those intended to be mirror imaged....

The 2SA1085 and 2SC2547 complementary pair (not even available when this was made and "obsolete", as in still fantastic but they don't make them any more, not as in that there are better more modern parts available. see my earlier post on this matter!), which Paul mentions are an interesting choice which, whilst I haven't done any analysis of it, I think see what he's probably getting at here...

They are not a fantastic choice for a MC head amp, not that bad either, (very good for a MM one though) but, bearing in mind how current starved the devices are in this application , they may well be a better compromise under these battery saving conditions than something ostensibly far more suitable for low noise with a source impedance as low as that of an MC cart. So probably a good shout but would need some measurements etc to confirm

To get to Marco's other point in the next post (i think) and take it further ().. the ultimate Lentek? would put the PCB in a much bigger box and powered by a lantern battery (or similar capacity of rechargeable's), this would allow the current to be upped to reduce the hiss by probably at least half, and I would hope down to about 1/4 of original.. the extra class a, whilst technically pretty irrelevant, may smooth and refine it in practice....the tantalum capacitors would be replaced by polypropylenes FAR too big to go in the original box (similar to but bigger than the ones you used in the 15XR rebuild Marco). Not cheap to do other than as a labour of love of course!

As to, whilst probably making it a much better unit overall, would it keep whatever "je ne sais quoi" it's reputed to have....? Anyone's guess I'm afraid.. not predictable by any theory or measurement..( calm down Marco ) it would need to be tried to find out but the above is about as far as one can go in taking a Lentek to the limit but not really changing that much of the original design.

[Paul Hynes]

Hi Marco,

Undoubtedly each of the components used in the Lentek circuit will have a sonic signature, but as Jez points out, changing components may upset the sonic balance and the only way to find out is to try changing components. Very time consuming, as some burn-in time should be allowed for each component for conclusive results.

I used the brown thick film resistors in many designs in the 1980s with good sonic results. I initially chose them because they were manufactured using non-magnetic materials and I preferred their sonic signature over the slightly grittier signature of the grey resistors, which, incidentally, had magnetic material in the end caps on the resistor body.

I also used the 2SC2547 and 2SA1085 in many designs. The 2SC2547 was used in an MC input phono stage and 8 were wired in parallel on each side of the input differential pair to reduce their overall noise contribution. This MC phono stage was astonishingly quiet and the low level information retrieval was exceptional. Unfortunately I have no idea how these transistors would compare sonically with the ones used in the Lentek.

[Marco]

Thanks for that, guys. It's all very useful info that I'll be bearing in mind for the future

Marco.

[Paul Hynes]

I have been playing around with the circuit using ZTX651 and ZTX751 in my plug-in breadboard. With circuit values readjusted to work with an Aerospace 6v spiral wound lead acid gel battery that I use on my test bench, the circuit shows good linearity and low noise with a voltage gain of 24 with the readjusted circuit values. There were no stability problems shown on the oscilloscope and I would expect decent performance from the waveforms present during testing.

The 6v battery has way more charge available than a PP3 battery so I also set the circuit up for 2.5ma quiescent current in the transistors and the circuit works well providing a voltage gain of 33 due to lower resistor values from the transistor emitters to the battery terminals.

I would be interested to hear what the circuit can do in my system and may, if time allows, prepare a circuit board layout for inclusion on my next circuit board order, probably in Feb/March. This will allow me to experiment with different components without repeated modification of one board, which can eventually lead to board damage.

[Marco]

Sounds very interesting, Paul. Please keep us posted!

Marco.

[Arkless Electronics]

I have been playing around with the circuit using ZTX651 and ZTX751 in my plug-in breadboard. With circuit values readjusted to work with an Aerospace 6v spiral wound lead acid gel battery that I use on my test bench, the circuit shows good linearity and low noise with a voltage gain of 24 with the readjusted circuit values. There were no stability problems shown on the oscilloscope and I would expect decent performance from the waveforms present during testing.

The 6v battery has way more charge available than a PP3 battery so I also set the circuit up for 2.5ma quiescent current in the transistors and the circuit works well providing a voltage gain of 33 due to lower resistor values from the transistor emitters to the battery terminals.

I would be interested to hear what the circuit can do in my system and may, if time allows, prepare a circuit board layout for inclusion on my next circuit board order, probably in Feb/March. This will allow me to experiment with different components without repeated modification of one board, which can eventually lead to board damage.

I thought those ZTX's would do the trick

I'll not be bothering with PCB's here.... If folks want a clone of it from me it will be on glass fibre veroboard or point to point wired on tag strip.