Input impedance

[Synchrodyne]

It's interesting that those networks present such a low impedance to the cartridge to reduce the amplitude down to MM levels (nominally 5mV or thereabouts) while simultaneously undoing the following RIAA EQ by making a big filter with the resistance and self-capacitance of the cartridge. Simple, but elegant

I imagine that four such networks could have been configured to cover a reasonable range of ceramic cartridges.  Then for example they could have been included on an additional four-way Quad 33 disc EQ board, in each case with gain adjusted accordingly.

Another example is the Hacker GP45, which uses the ACOS GP104. That one works into a 100k volume pot followed by a 2-transistor preamp that looks to have a "medium" input impedance; rather less than the Bush. Perhaps "high-10s" of k rather than 100s...

What's interesting about this one is the lack of EQ. With the Baxandall tone controls in their centre position, the amp is ruler-flat across the audio band. The test specification involves verifying this, then going on to verify the response with a test disc, which shows the the cartridge does produce a pretty flat response working into the ~100k load. My example has been up in the loft for some 5+ years, and needs a lot of work, so I can't say how it sounds with the tone controls in their centre position, but I wouldn't be surprised if they need to be turned up a tad. There's +/-12dB of range from them, so plenty compared to the simpler networks in the GP42/SRP51.

That lack of EQ is strange.  With a ceramic cartridge looking into around 100 k, one would expect the bass to be sloping away at 6 dB/8ve below some in-band low-to-mid frequency.  But I imagine that with this kind of unit, much of the final circuit tweaking was done empirically, with acoustic feedback considerations in mind, too.

As I said earlier, I abandoned ceramic cartridges when I was 12, so this has been very interesting for me. Especially the diversity of approaches now compiled in this thread. Something of an understatement I know, but the fact you can get such a good frequency response so easily is very fortunate for the equipment manufacturers who don't need to mess about with RIAA networks! Hard to imagine how the classic 2-valve player would have been feasible if it wasn't for the magic of ceramic

Indeed.  Also I think the self-equalized ceramic cartridge was important at budget end of the UK hi fi amplifier market during the 1960s, with makers such as Tripletone offering units primarily designed to work with such cartridges.  When transistors came along, Tripletone initially used a bootstrapped emitter follower to provide a 2M2 input impedance.

   

Then it changed to a fet input, I think (although I’m not 100% sure) using the Mullard TAA 320 IC, which was designed for the job.

   

Here is another approach from the early days of transistors:

   

I wonder if T1 was a microphone transformer in reverse?

But it's interesting to note that the output of the power amp is picked up by R15/17 and fed to the "Tape" socket. This means the recording will be affected by the settings of the tone controls, and will have any EQ needed to make the speaker sound OK. When playing back the recordings, the sound will travel through the amplifier for a second time, so will be double-equalised!

This seemed to be Bush’s preferred way of obtaining a tape recording output from its equipment.  For example both the VHF64 and EBS64 radio receivers obtained the tape output from a resistive divider across the output transformer secondary.  I guess that it was one way not to have to worry too much about what impedance the tape output was looking into.

Other makers did differently.  Ekco, with its A274, A277 and C273 FM-only receivers, tapped the tape output directly from the demodulator, just ahead of the gram input.  Hacker did the same with its Mayflower RV20.  With FM-only receivers, this was reasonably possible without undue adverse loading effects.

Murphy followed German practice, and used a high impedance potential divider after the demodulator, for example 3M3 over 220k in the case of the A674SR.

This illustrates German practice from the 1950s:

   

As best I can determine the objective was twofold.  Firstly the signal was reduced to around microphone level, and in the 1950s domestic tape recorders at least had a microphone input, if none others.  Secondly the high impedance of the divider precluded additional AC loading of the AM demodulator, and thus distortion due to an unfavourable AC-to-DC load ratio.  In situ the loading – mostly by the volume control – was usually about right, but with little tolerance for much additional loading.  This type of German interconnection morphed into the DIN “constant current” amplifier-to-tape recorder connection by losing the lower leg of the set’s divider, that instead being provided by the tape recorder input resistance.

The high impedance divider would also provide some loading “protection” for piezo cartridges connected to gram inputs, at least where these were ahead of the tape out take-off point, as might well be the case in radiograms.

Apparently BREMA issued a recommended standard for receiver/tape recorder interconnections, but I have not seen it.

   

Cheers,

Steve