Hoved-fi

Hello,

I was wondering if you could provide some advice.

I have a Bravo Audio Ocean headphone amplifier.

First thing, I have an issue with the balance, it is not centered. It is off to the left. How could I correct this?

Next, do you have any recommendations for modifications that will improve this amp? Should I replace the caps and the regulators and output drivers with better versions?

Do you know how I can make the cross talk modification (cutting a circuit trace) to this amp?

Also, on the back of the board is an LED. Why is this here?

Finally, do you know where I can find a schematic of this headphone amp? I can find Bravo V2 schematics, but not the ocean.

It seems this amp could perform well with some updates. Do you have any other comments or suggestions?

Best regards,
Alex

About the channel imbalance. Have you tried tube rolling?

I would recommend replacing the caps.

The Ocean is the same as the older Bravo types.
A very basic circuit 12AU7 circuit.
Just in a different enclosure.

The imbalance is indeed most likely the tube itself.
As these amplifier have no overall feedback the gain of each channel is determined by the gain of the tube.
Finding ones with matching values is not so easy (read more expensive).
Many of the tubes I have here have imbalances up to 2dB.

If swapping tubes doesn't resolve the issue perhaps adding a 'balance' potmeter or 2 trimmers in the input path could help.
A difference bigger than 2dB points to a tube that is either not well made or on it's way out.
Usually this last bit is accompanied by either distortion, a bias voltage being off or the tube being microphonic.

I would need close up pictures of the top side and bottom of the board to see if and where it differs from the 'standard' Bravo amps.

Similar mods as in the Bravo amps should work well.
It may be a good idea to replace the IRF630 for IRL510
I have no idea why they keep using those FETs.
Probably because the roll-off is associated with tube sound and they aim for the roll-off.

Replacing some caps may also be a wise thing.
105oC types could be a good idea to get better longevity.
Not all of these tube amps have a very long lifespan in stock condition.

Adding bias pots may also be a good idea.

It would appear as though the volpot is a linear type.
When the volpot works 'abrupt' in the bottom part of the travel and the volume hardly increases when turned up from '12 o'clock' to 5 o'clock' you could either fit a log pot or add 2 resistors and create a semi log volpot.

The LED on the bottom of the PCB provides a (cheap) reference voltage and is essential for operation (the front LED may not be).
If you want it off... paint it black or replace it with 3 diodes in series.

Hello,

Thank you for the great reply.

I have taken photos of the board, let me know if you need more detail.

I have swapped tubes (ShuGang 12AU7 vs Electro-Harmonix), and the balance issue remains. Maybe it is a crappy volpot?

Here are the photos:




Let me know what you think.

Regards,
Alex

Above is the exact schematic of the Bravo, except C3 is 1000uF/25V instead of 6800uF/25V for the 'usual' designs.
Also the LED marked 'blue' in the schematic is the red front facing LED in this amp.
Also this amp only has one ceramic 100nF cap (in parallel to the power cap) instead of 2 as mentioned in the schematic.
The schematic is of the 'standard' 12AU7 Bravo amp by the way.

Will draw the Ocean schematic myself as well one of these days.
Will be easier to read.

The volpot is linear.
If you find it adjusts poorly (reacts very fast in the minimal setting and almost nothing from halfway to full) an easy fix exists.

To check if the volpot causes the channel imbalance just temporarily connect the 2 middle pins on the volpot together (a drop of solder ?)
There are 2 'rows' of 3 pins. Connect the 2 middle pins of the 3pin row.
Now the amp is mono and thus L and R should be equally loud.
If not it must be either the tube or biassing.

when you have a voltmeter check the voltages from the + of each 1000uF capacitor to ground.
the ones next to the LM317, not the single one near the RCA sockets.

The crosstalk mod by cutting a trace needs to be performed UNDER the tube socket so it must be removed (which won't be easy) for this.

At least this PCB has 2 ground planes interconnected between layers.

I like to know the markings on the MOSFET (I suspect IRF630) and the value of the small caps next to the volpot (I assume 1uf)

Hello,

Bravo Audio Ocean:

The voltage regs are LM317T.

The mosfets are IRF630N.

The transistors are 2n3906 if that matters.

The caps by the volpot are 50V 1uf electrolytic.


Balance issue:

Going mono with the volpot short does not fix the balance issue.

The two caps, ~8.6V (across cap terminals) on the right side cap (top down photo), ~9.2V on the left side. The balance is off to the left side. I bet that explains balance issue?

That is about 6% difference, which corresponds to the ~6% I adjust the balance on the source (62L vs 66R out of 100 for a PC source to fiio kunlun).

That is 12dB though if I did the math right?



Bravo Ocean Ocean Suggested Mods:

Replace the IRF630 with IRL510

Update caps, higher temp. Higher farads or no?

Crosstalk mod. How to do? I have solder suckers, etc so I should be able to get the socket out.

Add bias adjustment. Two pots? Where do I install the bias pots?

Balance pot as well? Where to install?

Also, is there an easy way to make a "soft on" so the headphones don't pop on powerup?

Finally, add 2 resistors and create a semi log volpot? How to do?


Thanks for your help,
Alex

Here you are...

The original schematic and modifications: http://www.mediafire.com/view/tef7pnitixakzaa/Bravo_Ocean_schematic_modifications.pdf
(let me know if the link doesn't work, mediafire is acting weird again while posting this.

Schematic:


modified schematic:



The 1000uF across the red bottom mounted LED may be enough to lower the 'plop' on start up.
Maybe (can't try it here) you may also need to add a small 10M Ohm resistor from the collector of the 2N3906 to ground (one on each channel).

The bias pots may be enough to cure the balance issue as well.

When you have removed the tube socket take a picture of the top side and I can see where to cut traces and add wires for the crosstalk mod.