The SOHA needs only about 0.05Vp to reach full output into 24R. This why you don't have much volume control.

120R on the output will make the SOHA require about 0.3V to reach full output. This will be more of your volume control. I think you might have to try this to tell us how much it helps. Given that the volume control is logarithmic, it might help more than we think.

[Edited by runeight on 11-21-2005 at 09:04 AM.]

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quote:

[By runeight] The 300R goes between the wiper of the volume pot and the grid of the tube. It's called a grid stopper resistor and is there to prevent parasitic oscillation from the tube. It's probably not necessary in this amp, but most of the time we put them there anyway.

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is it ok to use 100R instead of 300R for grid stopper resisters?

Also, for reference, this diagram shows the changes we made to a few of component values for pho_boi's amp.

Basic Stoopid Updated

It's just two resistor values.

[Edited by runeight on 11-21-2005 at 08:53 AM.]

My tests (and Batman's) were carried out using a portable mp3 player as that's the point of this amp (sitting at a desk), and the player output was kept fairly low due to the gain of the 12AU7.

A 6922 would be even more problematic as its gain is higher still which is why we didn't use one of these "all time favourites" here.

It is good to know your (pho_boi) amp works now and sounds as we intended.
Can we take it as an independent opinion that the "Stoopid" ain't too shabby ? ..or not ? .. as the case may be.

Be very honest ... after a run-in period .. that is.

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quote:

[By pho_boi] By putting resistors in series with the output, will it let me use the full sweep of the volume pot without running into distortion? The resistors would decreases there sensitivity?

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pho boi, there is another solution for ur problem. i saw 47K resistors right before volume pot on MHHA scheme, so i posted a question in HAS what is the role of those resistors, and sijosae kindly told me a volumn pot works as a voltage divider and those resistors are supposed to change the division ratio thus reducing the input level.

suppose signal above +-1V will be clipped and u have a CD player with output level of 3V. u won't be able to turn ur volume above the 1/3 point w/o distortion. this has little to do with gain.

u put 10K before 10K pot, input level will be reduced to half. put 20K and 1/3. u could use output resistors as well at the same time, but i think this is bit more useful because u can expect exact result and it works on input side rather than output side of the amp.

ciao~

[Edited by ryanjun on 11-22-2005 at 06:07 AM.]

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quote:

[By ryanjun] another question.. what do 10k resistors right before stereo jacks do? what happens if i get rid of them?

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They are there as a safety feature at the output and to give the opamps just a little load when there are no headphone. But since the amp is DC coupled at the output they are not stricly necessary. You could eliminate them without problems.

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quote:

[By ryanjun] pho boi, there is another solution for ur problem. i saw 47K resistors right before volume pot on MHHA scheme

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Yes, this will work, but I'm generally not fan of making the input smaller as this tends to reduce the signal to noise ratio. But in this case, with so much input, it would probably work ok. Worth a try anyway.

I'll have to give a try with the resistors soon. I'll tell everyone how it goes.

The issue with my DAC outputting really high, my sijosae MHHA doesn't have any problems with the DAC and it uses the 6922.

The harshness could be the tube in this case.

I am wondering if your listening style and headphones need more power. The OPA2134 internally limits its output to 40mA (20mW continuous into 24R). This is typical of many of the opamp amplifers such as the cmoy.

If, after you add the resistors to get more volume control, you still don't have enough power you could do a couple of things.

You could put a BUF634 into the feedback loop of the opamp or you could add a discrete output stage such as the diamond buffer. I think that I posted some diagrams earlier in the thread for this.

I think that the MHHA uses a mosfet for the output stage. Is this correct? If so, it can deliver quite a bit more power than the OPA2134. Although the MHHA will dissipate a lot of idle power in the mosfets, regulators, and so forth.

So, if you really like the sound, there are things that can be done.

Don't know if this makes any real difference. Just desperately trying to help.

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quote:

[By Guitarsenal] Parts are arriving almost every day. My ALPS "BV" pot is here, and my tubes and sockets are here. Hopefully the bulk of the parts from Mouser will arrive today, and then the building begins. Can't wait to see the tube glowing!

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Good news! Can't wait to hear what you think.

In the meantime, be sure to look at the schematic that I posted just a few posts ago. This has some revised values based on pho_boi's build. Let me know if you can't find it.

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quote:

[By Steve Culton] body of the grid stopper resistor be physically placed very close to the grid pin on the tube socket.

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Yes, this is generally true. It matters more in some cases than in others. For tubes that really like to oscillate, the close the better. For the 12AU7 in this amp, it probably doesn't have to be that close.

I was looking at the transformer, and it has eight pins labeled 1 through 8. Can any of you tell me how to wire it?

If you've got the Triad, I've asked Batman for some help here because he's used this trafo before. But, you should have gotten a wiring diagram in the box with the trafo that shows you how to locate the pins and their numbers. If you can do that, then follow this diagram to wire it up.

Also, you did look at the schematic of a few pages back?

[Edited by runeight on 11-23-2005 at 04:07 PM.]

On the primary (115V) side, connect pins 1 nd 3 together and pins 2 an 4 together. One each of these pairs (it doesn't matter which) goes to each side of the AC line.

On the secondary side ci=onnect pins 6 and 7 together, these will form the center tap (ground) of the transformer. Pins 5 and 8 will supply the 30VAC to the powersupply.

Edit:Actually, Runeight's drawing above is correct.

Be very careful the pins on the trafo are not in numerical order on the trafo body. That is they aren't located in a row;1,2,3,4.

[Edited by Batman on 11-23-2005 at 04:23 PM.]

I don't have all of the parts for my power supply yet, but I just had to see the tube glow, so I wired up the heater part of the circuit:

I'm getting 34.8 volts from the transformer under load, and I'm seeing 12.6 volts across the tube heater filiment. Does that sound OK?

Those resistors really do get hot!

Is that really a 12AU7?

[Edited by runeight on 11-23-2005 at 11:36 PM.]

Hope that will work!

Before you actually turn the amp on with the tube in it, make sure that the trimpots are set for their maximum resistance.

Also, look at the changes we made for pho_boi's amp. They may be helpful in getting your amp to work first time.