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Old 15th Mar 2019, 8:17 am   #1
mole42uk
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Default Pre-amp for Linsley-Hood?

I’m building a 1969 Linsley-Hood power amplifier and wondered what pre-amp would suit it? I need inputs for my Shure V15, tuner, tape and CD.

The Mullard book springs to mind, but there must have been other equally contemporary circuits.
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Old 15th Mar 2019, 8:29 am   #2
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Default Re: Pre-amp for Linsley-Hood?

Depends on whether you want tone controls or not.
If not I would just use a volume control on the input of the Linsley-Hood with passive input switching for the tuner, tape and CD and then use a dedicated RIAA preamp for the Shure.
I have a design for such a preamp if you want, my own design using a passive, in line network for the RIAA response (not in the feedback loop as usual). Its what I use on my own system.

Peter
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Old 15th Mar 2019, 8:56 am   #3
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Default Re: Pre-amp for Linsley-Hood?

To keep with the Linsley-Hood theme, he did a preamp with his later (1973?) 75 watt amp. It's based on his "liniac" subcircuit.

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Old 15th Mar 2019, 10:02 am   #4
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Default Re: Pre-amp for Linsley-Hood?

I built his modular pre-amp that was featured in Wireless World, not too long after the class A 10W power amp design was published.

Ron
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Old 15th Mar 2019, 10:10 am   #5
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Default Re: Pre-amp for Linsley-Hood?

I was never satisfied with a couple of contemporary preamps which I built and used in front of my LH’s. I ended up with using designs from Elliott Sound Products (ESP), whose website includes the LH archive, using OPA2134 Opamps. Sacrilege maybe ! YMMV.

Ken
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Old 15th Mar 2019, 10:13 am   #6
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Default Re: Pre-amp for Linsley-Hood?

Back when I was a lad, I bought a kit for one of the LH power amps. Forget which of his designs it was. Must have been 1971 or thereabouts.

Built it, tested it - perfect. Then screwed the lid on. The lid had folds front and back. And the fold at the back was too long and shorted all the (TO3) power transistor collectors to ground. Bang.

I was both gutted and furious. Lots of hard won pocket money went into that darned thing, and the idiots who had designed the mechanics cannot possibly have done a trial assembly.

Of course, they did not want to know when I contacted them.
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Old 15th Mar 2019, 3:25 pm   #7
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Default Re: Pre-amp for Linsley-Hood?

Thanks! I’m looking at the Linsley-Hood pre-amps now, I rather like his approach using three transistors as a building block …
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Old 15th Mar 2019, 4:16 pm   #8
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Default Re: Pre-amp for Linsley-Hood?

The JLH modular pre-amp referred to in post #4 is in the July 69 edition of Wireless World, detailed here:-
http://www.keith-snook.info/wireless...Design-DCD.pdf

Rn
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Old 16th Mar 2019, 12:12 am   #9
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Default Re: Pre-amp for Linsley-Hood?

Quote:
Originally Posted by Craig Sawyers View Post
Back when I was a lad, I bought a kit for one of the LH power amps. Forget which of his designs it was. Must have been 1971 or thereabouts.
Just hauled the name out of a mysterious recess of my brain. Powertran or Powertron.

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Old 16th Mar 2019, 2:46 am   #10
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Default Re: Pre-amp for Linsley-Hood?

Quote:
Originally Posted by ronbryan View Post
The JLH modular pre-amp referred to in post #4 is in the July 69 edition of Wireless World, detailed here:-
http://www.keith-snook.info/wireless...Design-DCD.pdf
That used a shunt feedback RIAA stage, which H.P. Walker later demonstrated (with supporting mathematics) had a rather poor signal-to-noise ratio (14 dB worse) as compared with a series-feedback circuit. On that basis the H.P. Walker modules might represent a better starting point for suitable circuits from approximately the same period.

The H.P. Walker circuits may be found in WW 1971 May, p.221ff, and WW 1971 June, p.295ff. His article on low-noise amplifiers was in WW 1972 May p.233ff, and the resulting debate with JLH occupied WW 1972 August p.389, WW 1972 November p.520, WW 1973 January pp.11,12, and WW 1973 April pp.193,194. All worth reading…


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Old 16th Mar 2019, 2:49 am   #11
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Default Re: Pre-amp for Linsley-Hood?

Quote:
Originally Posted by Craig Sawyers View Post
Just hauled the name out of a mysterious recess of my brain. Powertran or Powertron.
Powertran, I believe.
I lusted after their kits as a teenager and a student, but they had gone once I started work and could afford one.
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Old 16th Mar 2019, 7:07 am   #12
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Default Re: Pre-amp for Linsley-Hood?

Quote:
Originally Posted by Synchrodyne View Post
Quote:
Originally Posted by ronbryan View Post
The JLH modular pre-amp referred to in post #4 is in the July 69 edition of Wireless World, detailed here:-
http://www.keith-snook.info/wireless...Design-DCD.pdf
That used a shunt feedback RIAA stage, which H.P. Walker later demonstrated (with supporting mathematics) had a rather poor signal-to-noise ratio (14 dB worse) as compared with a series-feedback circuit. On that basis the H.P. Walker modules might represent a better starting point for suitable circuits from approximately the same period.

The H.P. Walker circuits may be found in WW 1971 May, p.221ff, and WW 1971 June, p.295ff. His article on low-noise amplifiers was in WW 1972 May p.233ff, and the resulting debate with JLH occupied WW 1972 August p.389, WW 1972 November p.520, WW 1973 January pp.11,12, and WW 1973 April pp.193,194. All worth reading…


Cheers,
Thank you! More research is needed then… I look forward to reading the debate between HPW and JLH!!
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Old 16th Mar 2019, 5:02 pm   #13
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Default Re: Pre-amp for Linsley-Hood?

Didn't that get quite heated for the letters pages of WW?
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Old 16th Mar 2019, 6:45 pm   #14
Craig Sawyers
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Default Re: Pre-amp for Linsley-Hood?

No - WW was in the 1970's quite vocal on the letters page. There was an article from about that time titled something like "Displacement current does not exist" and putting forward the concept that any capacitor can be transformed into a transmission line structure, hence removing any need for displacement current.

The letters page ignited for months following that article
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Old 16th Mar 2019, 7:56 pm   #15
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Default Re: Pre-amp for Linsley-Hood?

It's not as though the maths is impossible..... Just hard work. Any circuit of resistors, capacitors and inductors can be modelled with a system of differential equations. But you don't even have to solve them the hard way! Using a computer, it's simple enough to run a numerical approximation, setting δt as small as you like, doing the calculations over and over again and adding up all the δV's and δI's; and then plot a graph to show what is happening over time. Basically it's like counting the squares on graph paper to get the area under a curve, as a way to an approximate solution to a differential equation; only with the computer doing all the work.
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Old 17th Mar 2019, 2:28 am   #16
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Default Re: Pre-amp for Linsley-Hood?

HPW seemed to be unafraid of the mathematics involved in the noise calculations. JLH eventually ran out of arguments in favour of his shunt feedback RIAA circuit.

Even back in the valve era, where shunt feedback around a pentode was not uncommon for the equalized input stage, it was known that the cartridge loading resistor could place a limit as to how far the noise floor could go. For example, in respect of its 22 control unit, Quad said, in respect of noise, “-80 dB or the where applicable, the equivalent noise of the pick-up load impedance at the input.”

In the valve era, using series feedback to avoid the input load resistor noise issue was more difficult, as a double triode with DC heating (at least on the input stage) would have been required. But in the solid-state era, series feedback was easily done with a two or three-transistor stage. That being so, why consider a shunt feedback equalized input stage? In the end, JLH could not provide a cogent argument for so doing.

HPW’s design also created another, but shorter debate about his use of bootstrapping in his tone control module. See WW 1972 May p.225 and WW 1972 September p.423.

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HPW was just preceded by Quilter, who published his bootstrapped tone control circuit in WW 1971 April, pp.199,200. This was intended to improve upon the Bailey 1966 circuit, which Quilter had commented upon in WW 1970 April p.172.

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Quilter was generally credited with this improvement – which became part of the “Bailey-Burrows-Quilter” control unit circuit, although it does appear to have had prior use, for example in the Quad 33:

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JLH had used a jfet to obtain high enough open loop gain in his 1969 modular design:

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Old 17th Mar 2019, 3:20 am   #17
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Default Re: Pre-amp for Linsley-Hood?

I'm not convinced the JFET gives any more gain. The unbootstrapped load in parallel with Zin of the follower times Gm. The bipolar beats it on Gm, but maybe he wanted to scale the Baxandall network for higher Z? It was in an era when transistors were still used sparingly. The 741 wasn't really up to the job, but later the NE5534 came along...

I notice JLH has his tape recorder output AFTER the volume and tone controls!

The WW series versus shunt debate was a little lively, but Hugh is a cool customer and sees maths as a useful tool. Noise analysis isn't terribly difficult and doesn't involve time-step calculations. If you want to see what Hugh did when he was getting serious, look up HP3745A in the HP journal.

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Old 17th Mar 2019, 8:59 am   #18
Craig Sawyers
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Default Re: Pre-amp for Linsley-Hood?

The original Cambridge Audio P40 used shunt feedback on a flat frequency response buffer input stage, with the volume control in the feedback arm. It does indeed sacrifice 14dB of S/N, but it meant that nothing in the preamp could overload before the power amp clipped.

That was important because a series feedback input stage with RIAA correction wrapped around it has very little loop gain left once the +20dB bass lift is in place. So warps in particular can push the input stage into overload.

When P40's were supplied for trial by the main Swedish radio station, they came back and asked what the frequency of the rumble filter was. There wasn't one. What was being interpreted as rumble in a conventional circuit was low frequency distortion artifacts resulting from warps in the record.

Which was actually a pretty radical design for the late 60's - a two transistor virtual earth shunt feedback buffer with volume control. And sure the noise was higher, but still OK as compared with noise from the vinyl itself.

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Old 17th Mar 2019, 9:50 am   #19
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Default Re: Pre-amp for Linsley-Hood?

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And sure the noise was higher, but still OK as compared with noise from the vinyl itself.
Craig
That is the key.

You can never make anything completely noise-free. All you have to do is make the noise low enough that it isn't significant compared to the noise implicit in the incoming signal.

The problem with RIAA input stages is that the record surface noise isn't there when the stylus is raised and people notice noise from the electronics without the surface noise to compare it against.

The P40 input stage was rather elegant in how it handled overload margin, but it had a couple of inconveniences that could have been fixed if needed. The Japanese designers took an alternate approach and used remarkably high supply voltages for their preamp stages.

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Old 20th Mar 2019, 11:17 pm   #20
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Default Re: Pre-amp for Linsley-Hood?

In his final letter in the WW correspondence series, 1973 April, p.193, HPW made the comment:

“With the shunt feedback arrangement I have often found that amplifier noise is audible above tape noise and surface noise on discs, and, with the new generation of "noise-reduced" recordings and low-output heads, I do not consider this a satisfactory situation as I believe audible background noise to be a source of listening fatigue.”

In his 1972 May article on low-noise amplifiers (p.233ff), HPW ran through the noise calculations for the shunt- and series-feedback cases. With perfect amplifiers, the shunt feedback arrangement gave an S/N ratio of 58.5 dB against a 2 mV input, the series-feedback arrangement 72 dB. He measured the 1969 JLH shunt-feedback design at 58 dB, and his own series-feedback design at 70 dB.

For its P40, Cambridge claimed 60 dB against 3 mV input, which would be 56.5 dB against 2 mV. Interestingly, the same 60 dB against 3 mV was claimed for the P50. This had a Darlington emitter follower at the input, which evidently did not provide any noise benefit, although presumably it allowed the use of lower impedances in the following virtual earth stage.

One might say that if you are going to use a shunt-feedback RIAA stage, then the JLH design was very good of its kind, getting close to the theoretical best signal-to-noise ratio. But that S/N was just a little short of what was desirable in practice.


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