eros hum and sound quality

[mkontor]

Well it all works seemingly as it should but I some issues:
there is a hum that is present even with the inputs shorted; no earthing arrangement does anything, and I know a bit about hum - it is internal to the amp
the ef86's are quite noisy - lots of tube rush (wind) and they are quite microphonic; I have the the Svetlana's supplied, some Siemens and Mullards.
The worst thing is the sound - too bassy and not enough  top end, very rolled off. Far too 'tubey', not very transparent. I can't believe this is how bottlehead wants this to sound, so I am hoping something is wrong with my work.

Have owners found a significant difference in overall tonal balance compared to their original decks (using the built in preamp)?
I don't mean, it's sweeter, more fluid, etc etc, I mean only in terms of tonal balance.
Cheers,
Mike

[Doc B.]

Need a lot more info, but based upon your description I would guess that the playback head output is relatively low, and you have cables between the head and the preamp that have high capacitance. What is the deck?

[Paul Birkeland]

The worst thing is the sound - too bassy and not enough  top end, very rolled off. Far too 'tubey', not very transparent. I can't believe this is how bottlehead wants this to sound, so I am hoping something is wrong with my work.

How did your voltage checks go?  Are you using the phono Eros or tape Eros? 

Generally, a frequency response anomoly like this would be the result of a mistake in constructing the EQ in the circuit, though there are other potential sources.

-PB

[mkontor]

All voltages check out fine.
It's not a cable problem as this hum exists with the inputs shorted, as I mentioned....
Using a Pioneer RT707 with relapped playback head (forward direction only) with very well shielded cable to gold RCA outputs

[mkontor]

I tested the unit with my turntable (Grado MM cartridge) and the output is higher but not significantly so. I know the EQ is different for phono but even so, the same sonic signature was there - bassy and rolled top end. It's not there is NO treble, but there is no sparkle, and the roll off is significant. The standard electronics in the Pioneer are better in every way, at this point.

[mkontor]

in fact the cable from the head are very low capacitance designed for phono lead (can den Hul D502II hybrid), and anyway high capacitance would not explain the bass boost.

I don't see how I could have got the installation of the EQ section wrong...it's a simple kit (I have assembled full blown amps in the pastt and make loudspeakers)

[mkontor]

in my previous reply I meant to write
' It's not that there is no treble....'

[Paul Birkeland]

I don't see how I could have got the installation of the EQ section wrong...it's a simple kit (I have assembled full blown amps in the pastt and make loudspeakers)

Nobody is above making mistakes!

Can you tell me the OB and Breg voltages on your PC boards?

Can you triple check that the 0.47uF and 0.047uF caps are in the correct position?

Having measured plenty of tape Eros units, the frequency response is quite flat and tipped up bass indicates an operational issue (or potentially a tape head issue, but you would also have this through the stock electronics).

[mkontor]

I did not mean to imply I was perfect (LOL) in fact I did make two mistakes which as you see in previous posts were rectified. But I have checked the construction and measurement many many times. Still, I am hoping for a mistake.

ALL voltages are exactly what the manual says they should be KREG is 1VDC breg is 100VDC, IA is 225VDC

Yes the caps are in the right place.

Any ideas about the hum?

[mkontor]

On page 62, there is an error - it says to mount the resistor from T111 to T10 - it should say T12, as shown in the picture; this was my first mistake,  i followed the instructions then got an incorrect resistance check on that channel (see another post)

On page 63 of the instructions you show 2 pictures; the bottom one shows two components on each side that are not in any of the other pictures, nor are they mentioned in the instructions; these are
a cap from T11 and T12 and another from T4 to T5
a blue resistor from T12 to ? picture is cut off and another from T4 to ?

I think this is the EQ part of the circuit so could be the cause of my unflat frequency response

[mkontor]

more about the caps shown on page 63 - these are 630V, and there are these in the parts list:

( ) 2 - 0.1 uF 630v metalized polypropylene capacitors ( F104K) or (0.1uFJ)
( ) 2

[Doc B.]

I'm not yet at the office, but when I get in I will check into those photos for you. It's been a while since I had my head under the hood of a tape Eros, so my recollection of the parts list is a little foggy and I need to get to all my documentation so I don't further confuse things. It is possible that we were trying to focus on some other aspect of the assembly in those photos besides the parts in question and we may have put a detail shot from a different preamp (i.e. the phono version) in because it was more clear. If they are only visible in one or two photos but not in the photo of the completed underside that is probably the case.

Just a few ideas in the meantime that might or might not help -

Re the hum, the circuit is very sensitive to magnetic fields from other pieces of gear. I have had a couple of occasions where 60Hz hum that seemed to be coming from an Eros itself was actually due to its sitting too close to the power transformer of a tape deck. This hum can be present with inputs shorted as it is coupled into the circuit components themselves.

Also you mention that hiss and hum are both present. That makes me wonder if the preamp you have the Eros connected to has a lot of gain. If the head output is relatively low (like under 2mVrms) it could be that the volume control needs to be turned up a lot and that your preamp is capable of amplifying the low noise floor of the Eros, which when greatly amplified does have a slight hiss component from the EF86s and a very tiny hum component. These aren't usually audible unless the volume is cranked way beyond comfortable listening levels, hence my wondering about the play head output level. Does it seem that you need to turn the volume setting much higher using the Eros than if you use the inboard electronics of the deck?

I'm not saying that either of these is the actual issue, it does sound like maybe there is some fault in the EQ circuit. But it's worth eliminating these things as possible contributors to the problem. 

[Doc B.]

It does indeed appear that the photos on the bottom of p63 and top of p64 are actually from the phono version of the manual. Anything other than the wires called out in those photos can be disregarded.

Have you tried substituting some different cables between the playback head and the Eros? Rolled off treble and tipped up bass are basically the same thing, as there is a limit to the amplification factor of the Eros and thus all EQ is really only subtractive, i.e., there is no real bass boost, only mid and or treble cut. Hence the influence of a cable interaction that rolls off the treble can seem like it also boosts the bass. I don't mean to seem stuck on this point and I am certainly willing to look at every possible reason for the issue. But I have had to deal with this exact issue at more than one trade show where an exhibitor has used an Eros with a high quality cable of their own choice and complained that the treble was rolled off, only to find that when we changed to a different cable everything was fine.

I am also not familiar with the head in an RT707, so I can't know for sure if its impedance is a good match to the loading we provide. That loading has been optimized for heads we are familiar with - Technics RS1500, Otari MX-5050, Studer, Ampex ATR, Nagra T Audio, Flux Magnetics extended response. Those for the most part are around 200mH inductance and a load of around 75K to 90K seems to work well. It is possible that a different load impedance could help a bit, but without any specifications it would have to be a cut and try type of adjustment. It's possible to replace the input load resistor with a 100K or 250K trim pot wired as a variable resistor to make this adjustable.

Of course all of this is long distance speculation. To best solve the mystery a response curve of the preamp would be the first, best step.

OK we have discussed checking the EQ to make sure it is correctly wired and that the caps values are correct. We have discussed head to preamp interconnect cable and head loading. We have discussed possible coupling of hum from outside sources. It could also be a tube issue, though that is typically a problem in one channel, no both. Substituting a different set of tubes might give some information. Also, the output impedance of the Eros is about 3K. If the load it is running into is very low, like 3k or less, or if the cables from the Eros to the next piece of gear are very long that could have some influence upon the perceived bass performance, i.e. it could sound loose or rolled off on top. If the load is above 10K or so and the cables are under 2M long this is probably not the issue.

[mkontor]

thank you for your long reply
when I said noise I meant to rush not hiss.. the preamp is quite noisy ... I did not have to turn the volume up very much at all... it's about the same same thing as direct out... the cables i used are standard cheap ones because I did not want to make good ones up yet.. anyway I am familiar with  off with cables and this is certainly not the case this roll off and bass boost is quite severe... I sent a unit to the guy that makes my valve amplifiers is to check out so I don't have it with me at the moment but obviously if he gets hum as well there is a problem with the unit.. He designs valve amplifiers and is very knowledgeable

[mkontor]

sorry was using phone to type and there are some errors in my reply

I meant to write tube rush nit his.... and that I sent the unit to my friend.... the other errors I think you can figure out