Opinions on an Eico ST-84 preamp

Hi,
I was up in the attic the other day and found this preamp.I don't remember buying it,but I must have.Can anyone tell me how it compares to the HF-85,
or Scott LC-21,or Fisher 400c preamps?

Eico st84 front.jpg Eico st84 top back.jpg 7a0ebd1104c6f1b88c195c7c486e2fed.jpg
 
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The biggest change over the HF-85 design was in the phono preamp circuits, which employ feedback around both stages in the 84, but only around the second stage in the 85. Other than that, the 84 is basically a copy of the 85, with the design errors of the 85 largely all corrected in the 84.

Dave
 
I remember in the ST70 integrated the BAL positions invert the left and right channels and add them together to null the output if the tone controls are set exactly the same,a unique feature.
 
Loudness fix is recommended on this one too - the volume is rather "hair trigger" without it. If you have a hum you can't seem to get rid of, put the bottom back on!
 
The Eico loudness switch "problem" was a problem for everybody except Eico. Eico intentionally designed the loudness circuit in all of their models so that there was a major volume reduction when the loudness circuit was engaged, since they believed that -- if used properly -- the loudness circuit would only be used during very low volume listening occasions, such as for the purposes of providing background music. Of course, that is in fact supposed to be the true intended use of any loudness function on audio equipment. Eico took that definition quite seriously though, and designed their loudness circuits to actually address the intended goal: When engaged, the volume level was significantly lowered (allowing much more low volume level flexibility with the level control), and the low frequencies were appropriately boosted -- exactly what you need to appropriately adjust the unit for low level background type listening.

The trouble is, nobody else took the definition of the loudness function as literally as Eico did, so that virtually everybody else made their design simply boost the lows (and usually the highs as well) as long as the level control was somewhere below about a 12 noon setting -- and had it all happen without a significantly change in volume level as the loudness circuit was engaged or deactivated. Against that, the Eico loudness approach -- although well intentioned, and even more in keeping with the intended used of such circuits -- simply came off as strange, weird, poorly designed, containing schematic errors, and a whole host of other accusations about what a problem it was -- but all as judged against the "norm" of other manufacturer's loudness circuit designs of course.

In the end, well intentioned as the design was, it was simply never accepted by the public, with virtually everyone today agreeing that -- for various reasons -- the circuit needs to be "fixed". To that end, I agree -- if only because today's vacuum tube audio enthusiast hardly use their prized piece of equipment to provide low level Muzak type background music, but rather, operates the equipment when they really want to LISTEN to the music, relegating a cheap boombox for actual background ambiance. Also, the idea "low level listening" by vacuum tube audio enthusiasts today likely represents a significantly higher (although still considered as low) listening level (as in foreground music) than what Eico was considering low level listening to be back in the day.

So, for a number of reasons, the circuit does need to be "fixed" today. However, the Eico engineers should be given some slack, as in fact, they were the ones who technically got it more right than anybody else did. In their mind (correctly), the equipment was high fidelity equipment, which was defined by one term in the day: accuracy. If you were running the equipment at "normal" listening levels, the loudness circuit was not supposed to be used in those occasions. If you did, it wasn't accurate, so they designed their equipment so that it basically didn't work at normal listening levels.

Of course today, the idea of strict accuracy has taken such a rear seat to everybody's own subjective definition of what the term means, so as to have basically fallen off the bus. Therefore, in today's listening environment, anything goes, which means if the loudness switch doesn't work at high volume levels, then it obviously needs to be "fixed". If any of them are still around, I'd sure hate to be the poor Eico engineer who designed those circuits!

Dave
 
Dave, Thank you very much for the explanation to a question I've had in my mind about the Eico "loudness" issue. Fritz
 
Something Dave didn't mention, and I only implied... the "loudness" network (the most commonly used one) effects the volume control curve - even in the "flat" or "off" position. It reduces the rate of change at lower levels, since that part of the control is shunted by a resistor. EICO's implementation resulted in the volume rising much faster as the control is advanced, UNLESS the "loudness" function is switched on. Most of us are used to a more gradual adjustment of volume and will prefer the "conventional circuit". If you have a power amp with an input level control, the EICO circuit may be fine, as you can set the useful range of the volume control wherever you like it.
 
Several eariler EICO products (the HF-52 mono integrated, e.g.) took a somewhat more elegant approach to the loudness imbroglio - providing separate level and loudness controls (pots) - the idea (not unique to EICO and even used later by companies such as Yamaha) was to adjust the VOLUME pot to the maximum normal level, and then use the LOUDNESS pot to reduce the listening level - all the while providing essentially smooth and continuous midrange attenuation.

Here's Yamaha's implementation. I honestly don't remember if the EICO circuit was, like Yamaha's, essentially a "classic" Fletcher-Munson curve set, or cut MR and HF (i.e., akin to LF only loudness compensation). My much-beloved HF-52, sadly, is long gone.

 
I prefer "normal" bass and treble tone controls and a volume control that just adjusts 'volume'. No loudness control at all. Thats just me.
Loudness is often read as 'volume'. loudness makes it sound louder ie. more volume.
 
The HF-52, and it's lower powered cousin the HF-20 both used the same dual control (level and loudness) scheme, that in fact operated on the Fletcher-Munson curve as provided by a pre-built Centralab "Comptrol" included with each kit, that was mounted in the loudness control position. I do not recall if the mid power HF-32 included this control or not. No doubt, by every appearance, Eico took the loudness issue more seriously than virtually anyone else back in the day.

Dave
 
Yes, the HF-32 does have the same loudness set-up as the HF-52 and HF-20. Works quite well in all 3 units IMO. If you set your target volume at 80ish db, where normal hearing is the flattest, and then set your volume for that level with the loudness at max. as you decrease the loudness it compensates well to my ears. YMMV and your speakers figure in as well. Adjust treble and bass to taste. My Pilot SP-215 has 4 loudness pre-sets and a normal setting and no explanation whatsoever of what or how it works! lol I do agree that on the HF-85 the loudness comp. is worthless. lol
 
I am sure I will rewire the loudness switch,There is plenty of room to work on this under the chassis,should I replace the PEC's with discreet parts?
Upskirt shot

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I'm working on a HF85 and made tone PECs from small 3/4" sq proto perf board. Not yet installed. I'll get a pic if you want.
 
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Yes I would like that,I would be interested if you can hear much difference when yours are done. I think Sam Googly was working pn PC boards to replace the PEC's also.
 
Several AK members have already built PC boards successfully and are using them in their HF-85 pre-amps.

I use my HF-85 almost daily to drive a pair of Heathkit 6bq5 mono amps and I am very happy with the way the discrete component PC replacement boards sound.

That ST-84 pre-amp is a nice find.:thmbsp:
 
It's a little frustrating,I have had this preamp for at least 15 years,and I forgot I had it,to tell the truth,I don't even remember buying it.I know I worked on it, as I jumpered the power switch originally(it must not work), soo... as I get older, old things are becoming new again,I hope that this is not a sign of things to come.
 
The reason you don't remember buying it is because its mine. I sent it to you to work on 15 years ago. Please send it back! :)

Dave
 
:lmao::lmao: It's only funny because I know that long ago I didn't do outside repairs,now if you said a laptop 6 months ago, you might have something.
 
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