eros hum and sound quality

The service manual is available online from "hifiengine"; I registered and downloaded it. The tape head preamp assembly has a very-low-frequency gain of about 1800 (about the same as Eros) and an input impedance of 100K, so I speculate the tape head is similar to others for which the Eros is designed. The equalization on that board looks to be similar as well. The head itself is part number RPB-041, but I can't find any specs. There have been a few threads on AudioKarma - I didn't see anything useful though.

Based on the above, I would expect a correctly-functioning Eros to provide nearly inaudible tube rush and an excellent frequency response, so long as its input is correct (the correct head coil, direct to the Eros and disconnected from anything else - there must be a switch in the deck for bi-directional operation?) and its output goes to an appropriate load. All these have I think been mentioned, but one at least of them still must be the culprit.

Be sure to give your friend the manual, which has the circuit as well as the pictures showing parts placement. The circuit has terminal numbers identified which is a big help.
 
Thanks for the offer to look at and fix the unit. I will let my friend have a go at at least finding the source of the hum and noise. He will also check the construction. As you say in your manual, a second pair of eyes...


He does not have a tape deck with direct head output, so will not be able to tell much about the sound, however you get a fair idea with a MM cartridge (turntable).

If he has no joy I will get him to send it to you.

As far as my Pioneer goes, the forward head has been directly wired to a pair of RCA sockets with extremely well shielded cable. It is not connected to the internal amps when I've tested the Eros.

Cheers,
Mike
 
When hooking up a turntable with MM cartridge, what should we expect to hear in terms of sound (leaving the EQ in the Eros unchanges , with NAB selected) - more or less treble than with tape, more or less bass than with tape?
I'm asking because my tech friend does not have a tape head output....
 
The treble turnover frequency for RIAA phono EQ starts a bit lower than NAB. So the treble from an LP will sound slightly bright through the Eros tape head preamp.
 
Well that is certainly not happening now, so there is definitely something wrong with the EQ...
I'll keep you posted....
 
As an aside, what are the main differences between the Eros and your $4000 tape preamp? Going from the description, it's the same design.
 
Hi Paul,
When you say the manual - is this the same as what came with the kit, because that has circuit diagrams in it as well....
Out of interest, my tech friend (Elson Silva, he builds the Cymer amplifiers) really likes the 5775 tube as an input tube. If we get the Eros up and running as it should, we will try this tube to replace the EF86. I have a pair of NOS Siemens and Mullard EF86's and one of each pair is very noisy (this is not the tube rush I refer to - it's there on the quiet tube; BTW it is there with the inputs shorted, as is the hum), so I am not a big fan of the EF86, so far....Elson says the American EF86's are much better mae.
the quietest ones were the ones supplied (Svetlana), but swapping the pair left with right make the left channel more noisy.
Cheers,
Mike
 
Paul - I misunderstood - the service manual you are referring to is the one for the Pioneer; I have this.

Anyway, the tape head is directly wired to gold RCA sockets; the forward head has been relapped, the reverse, erase, and record heads removed to provide a better tape contact with the forward head. IMO this deck sounds superb - it certainly beats vinyl in every way, with the best tapes (and I have some wicked vinyl and a great t/table).
 
The Tube Repro adds a trim-able transformer balanced output stage at +4dB, premium parts like Teflon caps Tantalum resistors and turret boards along with a rather spendy cabinet to house it all in. It's hand wired by PB.

I will let Paul weigh in on changing the input tube from an EF86 to a 5755 if he so desires. The circuit is direct coupled and thus there are a lot of interactions to consider in making such a change. A much less complex mod is to rewire the socket to use a 6BR7. However I think this getting way ahead of the game. The preamp should be verified to be working to spec before mods are done. Unfortunately we are a bit restricted in being able to help verify that unless we have the opportunity to look it over.
 
I agree it's getting ahead of the game - just wanted to mention it for your interest and because I was getting poor results with noisy EF86's.
I may know the source of the hum - the isloation transformer that powers all the plate amps that drive my bass speakers has a hum in it that on all the sources is very low in level, but as you say the Eros is sensitive to picking up hum. One thing I did not try is to turn this isolation transformer off or plug the Eros into another power point in the rooms (all the other gears including the Eros is plugged into a totally silent, better quality isolation transformer.
Elson has the unit now and if he gets no hum, that problem is solved.
 
It's a bigass, obscure rectifier. Hence I am pretty sure he means 5755. Kinda like a 12AT7.
 
Take a look in the Eros Phono file.  I have a thread listing the drop in replacements/equivalents for the two audio tubes in the Eros.  My particular favorite are the USSR Military surplus tubes.  They are more rugged than other designs.  That is for both the EF86 and the 6922. 

They also tend to be quieter as well.  The ones from the 60s and 70s are reported to be the best.  Most of the sellers will send you what you ask for.
 
Just to keep you updated, the Eros got lost on the way to my tech friend, hence the delay; it's on the way back to me and then I have to send it to him again, so I won't have anything to report until late next week.
 
I would suggest that you send it to Bottlehead instead.  They know the circuitry, have tape decks to test with therefore they can be more efficient locating your problem.
 
There seems to be a bit of this going around. I got call from a tech in Texas yesterday (Pete Nyquist, super nice gent!) who had been sent a deck we modded a couple of years ago. He told the client it should have gone to us because we designed the mods done, and when he called me and I concurred he already had it packed and just wanted our address.

I'll just suggest that perhaps the return of the shipment is a sign from the cosmos...
 
Hi Dan
I've got the unit back now and thought I would check it out again before sending it to my tech and found that in the process I have lost the instruction manual which I had copied to my laptop, which I had to restore, so lost the file. I also lost the CD!!!!!
Can you please send me the PDF? I have dropbox (mike@leaudio.com.au) or please burn an image and point me to it? It will take too long if you post me a new CD.

Meanwhile I measured the resistor in the EQ section and they are both 200K, from memory that's correct. Did some other measurements too, all OK (from memory and more to the point, equal in both channels).

The hum is still there, fairly low level, but my tech is an expert at making tube amps with no hum at all.

I hooked up my LP player again as my Pioneer is wired up as stock again and it's a pain to re-wire, and the sound is definitely too warm, lacking in bite and treble. It's much brighter, has more attack thru the phono stage, so there is something amiss...

Without the instructions or at least a circuit my tech will not be able to do much...

Cheers,
Mike
 
I promise we will give it a full exam and make sure it functions to exact spec.

 
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