eros hum and sound quality

[mkontor]

I have tried 3 brands of tubes all the same results... I have to say that the ef 86 is not a great tube.... there was a big difference in tube knows when swapping tubes left and right hand and between the brands

[mkontor]

could you let me know where the eq circuit is and where we could install some pots for the bass and a trebke section

[mkontor]

I hopped up my turntable with standard moving magnet cartridge and I had to turn the volume down just a little bit so the output of the pioneer is not that low.... of course the eq is different for phono but I still heard the same treble roll off and bass boost

[Doc B.]

Trying phono was a good idea. The EQ section is between the two stages. Your designer friend should be able to figure out the resistors to replace from the schematic supplied with the kit. But unless the parts are in the wrong places or for some reason the values are wrong all that changing to pots will do is take the curve away from the proper turnover frequency points. The accuracy of the Eros EQ curve is generally regarded as quite good compared to other products and thus it would seem there is something that is not the way it should be like an out of spec part or a miswire.

There seem to be a couple or three different things going on here and consequently I am a bit puzzled as to what the cause is. We would be happy to have a look at the preamp for you, measure the frequency response of the EQ circuits and check the function of the EQ and also look at the noise floor, and correct any issues free of charge.

[mkontor]

TX fitter all that, to save on shipping costs I will let my designer friend do this... it would be helpful helpful if you could tell me some short cuts like where to measure and what to measure... remember he does not have a test tape... however you can certainly check out everything else... I'm really hoping I have done something wrong because this unit really does not sound correct

[Paul Joppa]

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.

[mkontor]

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

[mkontor]

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....

[Doc B.]

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.

[mkontor]

Well that is certainly not happening now, so there is definitely something wrong with the EQ...
I'll keep you posted....

[mkontor]

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.

[mkontor]

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

[mkontor]

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).

[Doc B.]

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.

[mkontor]

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.