[diego]

Quote:

Originally Posted by Parafeed /img/forum/go_quote.gif I've just been chatting about about the EAR headphone amps with an owner who has read my comments. If I wasn't clear, I'm not suggesting anything negative about the EAR design or implementation. My "sources of hum" comments are generic - they could be applied to any tube amp, not necessarily the EAR. I don't own any Mapletree product, but have designed and built several very similar designs with a gain stage driving a cathode follower, transformer or autoformer parafeed coupled to phones. To also clarify my earlier comment about the Hammond 119A output transformers (used in the EAR design) being "hum magnets". This wasn't directed at the EAR amp, or meant to suggest that their placement in the EAR amp is less than optimal. This comment was based on my own experiences with their placement in an amp with a choke input PSU filter. Orientation was critical with the Hammonds (for lowest noise pick-up from the PSU choke), but not an issue using alternative iron - the Sowter 8665 or MQ TL-404's. PS. One last comment, (and I should probably stop digging), but .... I'm told the SE uses a SRPP input stage. I'm also told the filament voltage is DC, with one leg tied to PSU ground after the rectifier. The cathode of the upper triode is going to be at half the B+. The filament is at 0 volts. While the 12AX7 H->K rating is 200V and I expect that this is not exceeded in the EAR design, I wouldn't be surprised (based on my own experiences with the 12AX7 in SRPP), because of the H->K differential, a) that the circuit is picky regarding the noise floor depending on the 12AX7 used - sample to sample, and that b) without a positive bias on the filament supply, 12AX7's will have a propensity for going noisy over time. Again, I'm stating an opinion, based on what I've been told, not that I have the circuit diagram in front of me.

B+ at the input tubes is 265v, the tubes are DC heated at 6.3v and are not biased up. The HV supply and the heater supply share a common ground as you say too. This is the case for the prototype but I haven't seen the schematic of the final version.

 

[Parafeed]

Quote:

Originally Posted by diego /img/forum/go_quote.gif B+ at the input tubes is 265v, the tubes are DC heated at 6.3v and are not biased up. The HV supply and the heater supply share a common ground as you say too. This is the case for the prototype but I haven't seen the schematic of the final version.

Replied to by PM.

 

[Budley007]

Just for FYI purposes, my HD250 is dead quiet with Grado HF-2s. Even though low impedance cans may exacerbate an existing hum-problem, they "should" perform flawlessly with a HD250 by design.

 

[indydieselnut]

Also for FYI purposes, I feel like the VERY slight hum I have on s/n 3 has decreased in the past month. I don't think it's my hearing because I'm currently evaluating gear for a new speaker rig and feel like my hearing is more dialed in than ever.

I just sold my WA6 so I'm down to just the HD250 and the Amphora SE. I think those two amps cover all my bases...might actually be done for a while!

 

[peanuthead]

Quote:

Originally Posted by indydieselnut /img/forum/go_quote.gif ...might actually be done for a while!

Famous last words

 

[indydieselnut]

Quote:

Originally Posted by peanuthead /img/forum/go_quote.gif Famous last words

You know me too well Peanut

Alas, speakers are beckoning...

 

[rich1122]

Ok,

I've done some more investigating tonight, and it seems that my s/n # 9 has the same exact problem as mazinguerbeta's # 7.

I get a very audible hum on the left channel only on both my RS-1s and my UM3Xs (it's actually much louder on the UM3Xs). I can hear a barely distinguishable hum on the right channel (with the UM3Xs only). The hum does not get louder if I raise the volume.

I've disconnected all of the interconnects, and the hum persisted.

I've swapped the tubes, and the hum stayed on the left side.

I've tried a cheater plug and the hum persisted.

I've tried plugging the amp into a different electrical circuit, and the hum persisted.

Anyone have any ideas how to correct this?

If not, I guess I'll contact Dr. Lloyd.

Thanks,

Rich

 

[Tonski]

I've already been in contact with Lloyd about this. His reply seems to answer why there is an issue with the left channel hum.

"The left channel wire from the volume control to the grid of the left channel input stage is longer than that for the right channel (because of the location of the volume control) therefore it is possible for it to pick up some hum. I didn't hear anything with my Sennheisers and I didn't listen to the preamp outputs. If you take the bottom cover off, you can see an orange wire connected to the middle terminal at the rear section of the volume control. This runs to the resistor connected to pin 2 of the12AX7/5751 of the left channel. This can be replaced by a length of shielded cable with the shield connected to the ground pins of the volume control. A prepared piece of cable could be installed quite easily."

I am going to do this and I'll let you know what the out come is.

Tony

 

[Parafeed]

The wires from the volume pot to the driver grids don't look much different in length to me. I can't see how shielding the left hand channel wire is going to make any difference.......

 

[mazinguerbeta]

Has someone solved the problem of the noise?, thank you

 

[rich1122]

I've also been in contact with Dr. Lloyd, and he had the following recommendations:

"1. The left channel wire from the volume control to the grid of the left channel input stage is longer than that for the right channel (because of the location of the volume control) therefore it is possible for it to pick up some hum. I didn't hear anything with my Sennheisers and I didn't listen to the preamp outputs. If you take the bottom cover off, you can see an orange wire connected to the middle terminal at the rear section of the volume control. This runs to the resistor connected to pin 2 of the12AX7/5751 of the left channel. This can be replaced by a length of shielded cable with the shield connected to the ground pins of the volume control. A prepared piece of cable could be installed quite easily. Can you or a friend do this if I sent the cable? This modification would also reduce the hum at the preamp outputs.

2. The Grado phones appear to be quite sensitive and so the hum could be further reduced by attenuating the signal right at the headphone output. A 6 dB reduction can be achieved by inserting a 33 Ohm resistor in series with the 4 Ohm output lugs of the output transformers. You just need to remove the wire from the lug and solder a resistor to the lug and reconnect the wire to the other end of the resistor (for each channel). This would make your "loZ" output reduced in gain (and with hopefully no audible hum). It would not affect the "hiZ" output. This modification, would have no effect on the preamp output hum. You would find you need to turn the volume control up further for the same headphone loudness but since the hum is not sensitive to volume control position you should be able to compensate for the reduced gain with no increase in hum.

3. The gain of the amplification circuit is quite high for a line preamp (you probably find you don't have to turn up the volume control very high when you connect it as a preamp). We can throw away some of the gain and make the hum inaudible at the preamp outputs. This can most conveniently be accomplished with a pair of in-line adaptors that I can make up to go between the preamp output jacks and your interconnect. A 6 dB attenuation should make a big difference. With the adaptors connected to the jacks, this will also reduce the headphone output at both the hiZ and loZ outputs so reduce hum there as well.

With one or more of the three modifications I think the hum can be eliminated. I can send you a package with the cable, resistors, and adaptors and you can try the adaptors first to see if the preamp hum is eliminated (and note the effect on the headphone hum as well). You may not have to go further but you will have the parts if it is necessary."


Note that I haven't tried using the amp as a pre-amp, but it looks like he feels the hum issue exists on those outputs also.

I am going to give these fixes a try once I receive the goody bag of cables, resistors and adapters.

Rich

 

[mazinguerbeta]

Dr Lloyd messages


¨I have been trying out the fixes I described on unit 10 of the HD250 before I shipped it. The substitution of the shielded cable did nothing. I am not sure why the left channel is so much worse than the right in terms of hum. I tried adding more filter capacitance in the power supply but that sis nothing (there is plenty of filtering with the choke-capacitor filter). With the preamp output, I found that an attenuator of 10:1 (20 dB) made the hum pretty much inaudible with my system. There is still plenty of gain for use as a line preamp. I wired the attenuator resistors right at the output jacks but I can make in-line attenuators for you to simplify things. For the headphone output, I tried some cheap (but sensitive) headphones so I could hear the hum. I then put 27 Ohm carbon film resistors in series with the low impedance output and the hum was inaudible. I took photos of the resistors installed which will help you in making the addition. I think these two mods will solve the hum issue to the greatest extent that I can. I will get some gold-plated phono plugs into which I will put the attenuator resistors with a phono jack installed where the cable would normally come out. I will send you the 27 Ohm resistors and the attenuators¨.
Someone has received this and has verified the result? Has someone a scheme of the placement of the resistances?

 

[Parafeed]

If I were a betting man.... It's electromagnetic induction. The hum is being magnetically induced into the left hand channel Hammond 119DA output transformer. Could be proximity of mains wiring (perhaps to the front panel on/off switch - does that run alongside one of the output transformers) or it's being "radiated" from the mains transformer or choke and "picked up" by the left channel 119DA.

 

[rich1122]

I am still waiting for the attenuators and resistors from Dr. Lloyd. Has anyone received theirs yet? If so, did it work?

 

[mazinguerbeta]

I have received it now but the modifications has not made yet.