Modifications to maximize sound quality of studer a820 for repro only – page 2 electricity deregulation map

I’m going to break my response into 3 categories: balanced vs unbalanced – object to follow; cable length (to which I will add “terminations”) and head amplification, with your referencing the fixed gain playback head preamp in the Studer A8XX recorder headblocks.

Balanced vs unbalanced first. Let me start by stating what we already know; a tape head is an inherently balanced device – just like a phono cartridge. What this means is that the two wires leaving each coil inside both devices are both “above ground”, with a potential difference across and opposing polarity between each other. Here, I’m going to leave the tape world for now and concentrate on the phono/LP world. I can do this because a typical PB head is electrically similar to a MM cartridge. Once the signal from a phono cartridge leaves its “balanced world”, things get interesting. You know that at the end of the shielded cable coming from 99+% of all turntable/tonearms (going to use TT from now on) is an RCA with a hot and ground. The ground becomes a common point (let’s call it signal ground) in the amplification chain and in most cases follows through all the way to the speaker’s minus terminal. So the balanced output from the cartridge has been unbalanced – by which I mean that one side is now referenced somehow to signal ground. This is done either right at the wires connecting to the cartridge or where those fine wires connect to the interconnects terminating in the RCA’s on the other end.

In a TRULY balanced world, what I will call the opposing signals emerging from the cartridge would be amplified / attenuated / equalized SEPARATELY all the way through to the speaker – with the speaker also operating in a BALANCED way. This NEVER happens. Somewhere along the line a balanced to unbalanced “conversion” is made. Name a TT that outputs a balanced signal and a preamp that accepts its balanced LOW-level signal. I know that they have been made in the past though I’m not aware of any now. What is out there are many (certainly most hi-end) preamps that may have balanced HIGH-level balanced inputs and outputs, and amps with balanced inputs. And the outputs from all the amps I’m aware of are UNBALANCED. In fact most speakers are unbalanced with one side of the crossover network for each driver attached to a common point (the negative terminal). Heck, a loudspeaker driver is an inherently balanced device. If we were REALLY TRUE TO THE BALANCED CAUSE, each loudspeaker driver would have a balanced crossover (I believe the term for these are “series” vs “parallel” crossover networks).

Now, assuming that you do have a preamp that offers a balanced low-level input and output, how does it accomplish this? The most complex method is to have FULLY BALANCED electronic amplification, which means you have two separate amplification paths for the plus and minus signals for each channel (a total of four parallel paths for stereo). This has been done by a few manufacturers although the equalization network have to be more complicated and controlling the volume more tricky – and twice as expensive? Most of the manufacturers take what I will call the “PSEUDO-BALANCED “approach whereby the preamp accepts a balanced signal and IMMEDIATELY un-balances it either through the use of an input transformer or electronically using a differential amplifier. Downstream, the majority of the amplification / equalization / attenuation is unbalanced; and then at the output, a device such as an output transformer or an electronic phase inverter “recreates” a balanced signal.

Back to the tape world and commercial recorders: I’ve seen VERY few playback preamp circuits that actually accept a balanced input from the PB head. The MCI’s and later Lyrec Frieda’s actually have electronic differential input amplification. One version of the Studer A80 (with a really low inductance PB head) and the Ampex MR-70 used an input transformer on the PB card – not sure of the ATR. The playback cards of most A80’s have unbalanced inputs and even though they use a two-conductor shielded cable from the PB head, one conductor (common) is grounded on the playback card. The fixed gain preamp in the A810/2 (and I assume the other A8XXX models) have unbalanced gain, with what I’ll call the common wire from the head “floating” in the headblock amp but grounded on the repro card. In ALL cases these preamps use UNBALANCED amplification / equalization. Most use an output transformer to derive their balanced output signal. There may be a model that uses an output electronic phase inverter but I can’t remember.

PART 2 – cable length. Yes, we are talking a “tiny playback head signal (running into) a meter or two of wire” before entering the preamp. Again, let’s compare this situation to the TT / LP / phono world. Think of the TOTAL cable distance and number of interconnections in an LP setup between the phono cartridge, tonearm and the RCA’s on the end of the cable that plugs into your preamp. You probably have at least a meter of total length, at least two different types of wire/cables and maybe two interconnections (clips on the cartridge, connector on the bottom of the tonearm) plus the eutectic solder joints. Heck there is probably 12-18 inches of tiny wire inside the tone arm alone. AND, consider what’s going on inside your preamp before the signal gets to the first amplifying tube or transistor – additional wire and/or PC board trace(s), cartridge loading components along with how those all are wired and switched.

No one in the TT world seems to complain about all this cable / interconnection? So in our tape world why not compare this to a meter or so of good quality, single conductor, shielded cable soldered directly to the tape head terminals. Problem? Haven’t had or heard of one yet with my outboard pre’s.

PART 3 – Head amp. Let me return to my TT analogy. How many folks are you aware of that have put some kind of amplifying stage closer to the cartridge like inside the turntable base or somewhere in the tonearm?? I’m not saying that it might not be a good idea but nobody in that world is doing it. Studer incorporated it into their A8XXX series for some good reason – I’d opine to possibly “offload” the work (and signal levels) that their reproduce card has to do. However, I’m not aware of any other manufacturer that did this. Again, it might be a good idea BUT there is barely enough room for an IC, and no to room do it discretely. And if you have to, make sure it’s fed with additional low-noise power regulation.