Tube Amp with Balanced Inputs Dilemma

dmatt · « **on:** 29 Jun 2009, 01:49 am »

Hi there,

I hope the tube brain trust can help me with a dilemma.

I recently acquired a used Melody I2A3 integrated amp. Popped it into my system and loved the improvement over its younger brother (Melody I880). Until I heard a weird random sound -- kind of like a 'braat'.

Making a long story short, on certain low notes (typically a plucked bass string), instead of the woofer sounding a 'bumm-bumm' it gives me a 'braat-braat'. Only on certain low notes. The best example is the song Hear We Go Again by Ray Charles and Norah Jones on the CD Genius Loves Company. The first note from the bass guitar is I think a low B flat and 'braat'. Every time that note is played -- the same thing happens. On bass guitar notes above that one -- no problem.

I have listened to this song a lot and have never heard this problem, so I started troubleshooting in my limited way.

The source is a Logitech Duet Receiver with ART upgrades playing through SPDIF to a Channel Island VDA2 DAC with outboard VAC1 power supply, then through CI VRX1 Harmonic Tech balanced cords to the amp. From the amp I use 3 feet of 14 gauge copper to a pair of Onix Ref1s. The speakers are rated at 4 ohms so I use the 4 ohm tap on the amp.

What I have found out:

Both speakers make the same sound
I don't have this problem when I use the I880 instead of I2A3, even using balanced inputs
I checked the bias on the I2A3 amp - instead of 1.5 volts, the bias was 1.75. I reset the bias thinking that was it. Unfortunately the problem is still there
It does not happen if I use single end RCA cords instead of balanced between the DAC and I2A3 amp

With the observations above, I am guessing it has something to do with the DAC -- balanced inputs -- amp -- 4 ohm speaker impedance interaction (yes, I am Master of the Obvious

). But putting the Melody I880 back in the system or changing to single end RCAs makes the problem go away. Any advice on what is happening and how to fix it?

Thanks in advance.

David

Niteshade · « **Reply #1 on:** 29 Jun 2009, 02:35 am »

Hello-

Have any mods been made to it?

Blair

dmatt · « **Reply #2 on:** 29 Jun 2009, 02:43 am »

Blair,

Thanks for replying.

No, both I2A3 and I880 are stock, no modifications.

David

Niteshade · « **Reply #3 on:** 29 Jun 2009, 02:50 am »

There is no easy answer...yet.

Can you try another source with a balanced output? Do you have schematics or know where they are for the working & troubled amp?

Blair

dmatt · « **Reply #4 on:** 29 Jun 2009, 04:37 am »

Blair,

I can try my other DAC (balanced) to tomorrow. I also have the circuit diagram to the I2A3. How do I post a picture on AudioCircle? Is there a reference section?

Thanks again,

David

FullRangeMan · « **Reply #5 on:** 29 Jun 2009, 05:26 am »

Quote from: dmatt on 29 Jun 2009, 04:37 am

How do I post a picture on AudioCircle? Is there a reference section?
David

1- There is a button Create a Gallery or something like it, in one page of your Profile, after creating your gallery go to step 2.
2- Edit or create the message that you want post the image, and just click the button insert image (first button on left,
second line, below the B (BOLD))
3- Will open a new window for you search the image (JPEG or GIF), click the file button and after locating the image click upload image
and the image will be posted after your OK. Like this:

srb · « **Reply #6 on:** 29 Jun 2009, 06:05 am »

Quote from: FULLRANGEMAN on 29 Jun 2009, 05:26 am

1- There is a button Create a Gallery or something like it, in one page of your Profile[/], after creating your gallery go to step 2.
2- Edit or create the message that you want post the image, and just click the button insert image (first button on left, second line, below the B (BOLD))
3- Will open a new window for you search the image (JPEG or GIF), click the file button and after locating the image click upload image [/]and the image will be posted after your OK. Like this:

Don't mean to sidetrack this thread, but I just click the insert image button then browse to the file on my hard drive and upload.
No gallery involved or anything. Was the gallery required to accomplish this previously, and is no longer needed on the new site/software?

Steve

FullRangeMan · « **Reply #7 on:** 29 Jun 2009, 06:47 am »

Thankyou for the information.

dmatt · « **Reply #8 on:** 29 Jun 2009, 03:45 pm »

Thanks for the info! Here is the I2A3 schematic:

(Please someone let me know if the Chinese characters say "Do Not Distribute")

Niteshade · « **Reply #9 on:** 29 Jun 2009, 04:17 pm »

Unless I am missing something, that looks like an unbalanced input. I do not see anything for a balanced input.

dmatt · « **Reply #10 on:** 29 Jun 2009, 04:26 pm »

Blair, so if that is the case, the XLR inputs are there more for convenience than anything else.

That would also explain why the volume is the same whether I use SE or Balanced. I thought the gain of a balanced input is higher (looking at the output specs of the Channel Island DAC: Output Level: 2.25vRMS (Single-ended), 4.5vRMS (Balanced)).

Is there a way to confirm this visually by looking at the leads coming off the balanced inputs vs. the SE inputs?

David

richidoo · « **Reply #11 on:** 29 Jun 2009, 10:03 pm »

I think that this input stage is SE and the driver/output stages are balanced. The XLR input is not documented on this schematic. According to the 6moons review, the XLR input is an afterthought for US importer only. Probably the XLR jack is connected directly to the single ended input stage, dropping the other balanced leg. This should not be a problem unless there is a bad connection in the XLR wiring, or more likely the DAC output stage is expecting both legs to have similar load, and is clipping as described here.

Both amps are described by the mfg as Balanced design. Maybe the i880 is fully balanced, including the input stage, but that requires double the # of triodes. Can you find schematic of i880?

Beautiful amp, btw....

http://www.ciaudio.com/vda2.html
http://www.melodyhifi.com/I2A3.html
http://www.melodyhifi.com/I880.html
http://www.6moons.com/audioreviews/melody3/i2a3.html

Niteshade · « **Reply #12 on:** 29 Jun 2009, 10:26 pm »

Sounds like a logical explanation to me. Since both channels are doing it, I doubt there is a malfunction- unless this is a PC board amp and a common ground run is faulty between the XLR jacks or something like that. Seems like there would be some noise (hum, etc..) in the amp if there was a damaged contact or wire at the input section.

santacore · « **Reply #13 on:** 29 Jun 2009, 11:49 pm »

I too own this amp. Although I haven't used the balanced input in a while, I believe the volume was louder whenever I used it. The same was the case with the I880 that I also own.

Bill Baker · « **Reply #14 on:** 30 Jun 2009, 12:46 am »

What you have according to the schematic is a SE input. One tube, one grid. There are a few things to consider. 1) The schematic does not indicate the US version 2) The XLR inputs are still only wired for SE 3) There is a small transformer being used for the balanced input.

dmatt · « **Reply #15 on:** 30 Jun 2009, 04:24 am »

Thanks for all the responses.

I pulled the bottom off the I2A3 and took some images. You can see that the XLR jacks have three wires exiting (red, white, black).

These go to two little black boxes, one for each channel (flight data recorders?

).

I removed the screws from one, thinking that the box was a cover, but it is a potted mystery component

-- red/white/black wires go in and braided line exits.

The braided line is exactly like the lines coming off the SE RCA jacks. All are routed along the side of the case to the source switch on the front (you can see this in the top picture).

Does this help?

Thanks again for the analysis, guys. I really like the amp, I'd love to figure out how to lose the distortion.

David

dmatt · « **Reply #16 on:** 30 Jun 2009, 05:01 am »

Based on what the innards showed me and your feedback, I'm concluding:

1) The schematics I posted are for the non-U.S. amp (no XLR)
2) The U.S. amp has XLRs
3) The XLR connectors make use of all three conductors (balanced)
4) The balanced input goes to a (?) and is then sent to the input switch
5) I have to vacuum the amp before I put the bottom back on

If anyone thinks otherwise, please feel free to chime in. I'm making it up as I go along. And what is underneath the epoxy of the black box?

My next act is to try my Transporter with the amp, both balanced and SE. From the Logitec specs page, it looks like the output of the Transporter (3Vrms) is lower than the output of the CI DAC (4.5Vrms). Maybe that will make the difference.

But I still don't know why the I2A3 would be sensitive to the CI DAC output and the I880 isn't. I think the two amps use the same front end (same tubes, layout, etc.). I suppose I could take the bottom off the I880 to be sure.

And Richidoo, thanks for the link to the clipping page. I understood maybe half of it. I might have to read through it a couple of times. Unfortunately I don't have the I880 schematic for comparison, although I am willing to pull the bottom off it and take pictures. And you're right, the I2A3 is a beautiful amp. As much as I like the I880, I really like the I2A3. I suppose Santacore has an opinion on that as well, the lucky guy has both.

Thanks again (now where is the brush attachment to the shop vac?

)

David

santacore · « **Reply #17 on:** 30 Jun 2009, 06:07 am »

Quote

As much as I like the I880, I really like the I2A3. I suppose Santacore has an opinion on that as well, the lucky guy has both.

Yep, I do indeed have both(anyone need an I880?). They are each great in their own way, but in the end I preferred the I2A3. It's more refined sounding and has better bass, especially with my Omega's. It also costs more then the I880 so you get what you pay for. Overall I've been extremely pleased with both Melody amps. The build and parts quality are both very high. They are extremely musical and are the quietist tube amps I've every owned. Great stuff!!

dmatt · « **Reply #18 on:** 30 Jun 2009, 06:27 am »

Quote from: santacore on 30 Jun 2009, 06:07 am

... and are the quietist tube amps I've every owned.

I forgot to mention that, thanks for reminding me. Both amps are very quiet, no hum or hiss unless at full gain with my ear against the driver (and that always makes me a little nervous).

Bill Baker · « **Reply #19 on:** 30 Jun 2009, 11:38 am »

Based on the picture of the innards, you do indeed have a balanced units. As cool as it would be to have flight data recorders in a tube amp (in case of a crash), those are actually balanced transformers. The quality is unknown but at least you know what's inside.