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Possible group buy of Lundahl input/phase splitter transformers.

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Hi all,
I am currently dragging together parts for a Tabor clone and mainly need a nice input/phase splitter transformer. I would also like to upgrade my 6080 amp with currently uses toroidals as parafeed phase splitters.
Whilst scanning Ebay I came across this auction;


This has 16 x Lundahl LL1517. The 1517 is described as a linestage output transformer/splitter, but to my mind that simply means that its a bit beefier and a bit more lossy than the traditional interstage transformer (am I wrong ??). Should make an excellent candidate for a parafeed interstage and has excellent specs.

Anyway I need four transformers and if I go for this auction I will get 16, which makes 5 spare pairs. Once postage and VAT was factored in the auction should cost about £143.00, which would give a price of about £18.00 per pair + onward postage. The same transformers from Lundahl would cost about £100.00 a pair + postage.

So if anyone is interested in going ahead with this get in touch and if there is enough interest I will arrange a group buy. If there is more than 5 people interested there are two other auctions up at the same time, so a potential of 20 pairs in all.

I have been doing some extensive searching on this transformer. There is precious little useful information out there. Two things I have established is;

1- it is specifically designed for solid state output duties, which means that it expects a very low impedance drive. In this case many tubes might not satisfy. It also means that it it is designed to drive a 600ohm input, which might means that the need for low impedance drive may be much relaxed when presented with a >10K load. Would ringing become an issue then ??
I have heard of it been used in the "SlowBlow" Mic preamp (a SRPP design) as an output transformer, so it seems it can be done.

2- in terms of voltage capability the data sheet says
Max output level before saturation (sec. in series 600ohm) +24dBU @ 30hz.
dBU relative to what ???

The LL1540 is a line input transformer, so will be designed for much lower drive levels than the LL1517.

So there are still a few questions to be resolved before pushing on. We have 7days to find the answers so there is no panic.
Can I suggest that if I do go ahead with this I will be reluctant to order more than one set for braking, so maybe someone in America could arrange for another set to go that way directly.

Shoog said:
Max output level before saturation (sec. in series 600ohm) +24dBU @ 30hz.
dBU relative to what ???

From Wikipedia
In professional audio, a popular unit is the dBu (see below for all the units). The "u" stands for "unloaded", and was probably chosen to be similar to lowercase "v", as dBv was the older name for the same thing. It was changed to avoid confusion with dBV. This unit (dBu) is an RMS measurement of voltage which uses as its reference 0.775 VRMS. Chosen for historical reasons, it is the voltage level at which you get 1 mW of power in a 600 ohm resistor, which used to be the standard reference impedance in almost all professional low impedance audio circuits.
The definition of a dBu given by Wiki, and used in the context of Lundahl is;

dB(0.775 VRMS) — voltage relative to 0.775 volts.[4] Originally dBv, it was changed to dBu to avoid confusion with dBV.[citation needed] The "v" comes from "volt", while "u" comes from "unloaded". dBu can be used regardless of impedance, but is derived from a 600 Ω load dissipating 0 dBm (1 mW). Compare ambiguous use of dBu in radio engineering.

So the base reference voltage of 0dBu seems to be 0.775V, or one mW across our 600R load. Unfortunately that doesn't tell us much about saturation with higher impedance loads.

My interpretation would be that into a 600ohm load it should cope with about 10 V rms. This gives a power handling of about 166mW. There will be a designed maximum voltage handling capability which seems impossible to derive from the information given !!! Is it safe to assume that that is 10Vrms.

Anyone good at maths and transformer theory..

Thanks for the input.

dBu is the voltage equivalent to dBm, which is 1mW into 600 ohms. (A bit of Ohm's law to do...)

That is what I meant in another thread to you, that you'll be using it at a much lower level.

Your original explanation becomes much clearer, and the OEP transformer looks much more appealing.
So in the context of an input phase splitter the LL1517 would be more than adequate.
In the context of an interstage phase splitter, presented with a 30Vrms, I have my doubts.

The OEP's would be front end phase splitters, so would be perfect. I never expected them to work as interstagers, I was hoping the LL1517 would do that, but unfortunately I don't think that will work. either

We need to take stock, who would still be interested in the LL1517 simply as input transformers or output transformers.

Hi all,
There is 1 day 19hrs to run on this auction.
I would still like one pair.
dsavitsk would like one pair.
James would like 8 pairs (one whole board).
Planet IX would like 8 pairs (one board).

So the figures don't quite add up.
Either James and/or Planet IX need to accept 7 pairs to let both/either myself and dsavitsk have our pairs. Another possablility is for myself and dsavitsk to take four pairs each (still a bargain) and I would have to disappoint Planet IX.

If a compromise can't be reached then I can't go ahead with the purchase. Lets me know how you all feel by tomorrow and I will make the final decision.



Ps. By the way dsavitsk, can you get Euros or Stirling - since I have no use for Dollars.
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