[mt877]

I posted this in the Post pics of your builds.... thread. I thought I should create a new thread in case other people wanted to build this amp as well.

A fun build. Oatley K272C-NEW with Burson V6 Vivid op-amp.

I gave my DIY amp a name which honors our Aussie friends... I present the "Koala Amp".

Very nice! Tube based SE and OTL ?

Yep, tube hybrid. Is SE, coupling cap at the V6 Vivid L/R outputs. I included a 4.4mm socket for convenience to plug in some of my 4.4mm terminated head gear. Drives my DIY 500Ω flat head earbuds to perfection. I have some 600Ω transducers as well, but haven't built those earbuds yet. It will definitely drive the 600's very well too.

I am interested, where can I source these stuff ? thank you

Oatley Electronics K272C-NEW kit.
The kit ships from Australia and contains the board, Raytheon JAN6418 sub-miniature pentodes, NE5532P op-amp and loose components.
Look for the link of the product page for PDF notes, that will have some assembly tips and the schematic.

Burson Audio Supreme Sound Opamp V6 (Vivid Dual x 1)
Ok, you can tell from my pics that I didn't use the NE5532P Op-amp... went for the big gun Burson V6 Vivid.

SENRISE Aluminum Project Box, 43x78x140mm
From Amazon. If you go with the Burson V6, this box is a must. Many boxes I looked at didn't have the height clearance needed for the Burson V6 Op-amp.

APIELE 12mm Latching Push Button Switch SPST
From Amazon. Battery on/off switch on back panel. This switch has a ring LED, but I didn't use the LED and cut the LED wires off. It is a nice compact switch and looks nice.

BusBoard Prototype Systems Adhesive Standoffs
From Amazon. You don't see these in my picture. These standoffs are used to secure the board to the chassis.

18650 5000mAh Lith-ion Rechargeable Battery
From Amazon.

PNGKNYOCN 4.4mm Balanced Replacement Jack
From Amazon. This is really a replacement jack for a cable, but I used it as a bulkhead connector... got to use some ingenuity sometimes.

Some of the other parts I already had on hand. The USB C battery charger board and 18650 battery holder can be found on AliExpress.

For the volume pot I used a dual gang 50k audio taper (log) pot with a switch to turn on / off the amp.

Hope this helps you get started on this project.

Edit: Forgot some info. The Oatley kit doesn't come with an 8 pin DIP socket. They expect you to solder the Op-amp directly to the board. Make sure you have an 8 pin DIP socket. If you look at my picture, you see 2 green capacitors behind the op-amp, there are 3 caps, one is hidden behind the op-amp so you don't see it. Anyway to fit the Burson V6 you need to offset bend the leads of the center capacitor so that the cap is a little further toward the back of the board. If you don't do that there won't be enough space for the Burson to plug into the DIP socket.

Behind the capacitors you see a small board standing on edge. That is the 18vdc buck-boost board. If you mount that board flat, you won't be able to adjust the trim pot for the 18vdc because a capacitor will be in the way. I mounted the board on it's edge with the trim pot screw facing upward. Makes adjusting the 18vdc much easier.

Edit2: Added this picture for a better view of the board.

 

[mt877]

Load resistor	Left Vrms	Right Vrms	Output Power per channel (V x V)/R = P (in mW)
15.7Ω (Closest value to 16Ω)	2.653	2.640	445.61
33.5Ω (Closest value to 32Ω)	2.772	2.745	229.37
82Ω (Closest value to 80Ω)	2.835	2.794	98
150Ω	2.851	2.812	54.18
300Ω	2.864	2.823	27.32
580Ω (Closest value to 600Ω)	2.867	2.827	14.17
1000Ω	2.866	2.823	8.19
Output Impedance = 1Ω

Procedure:
Input: 1khz sine wave @ 1Vrms (2.83Vpp)
Connect left and right 15.7Ω (16Ω) loads
Adjust internal gain pots for equal (matching) maximum sine wave output without clipping.
Measure Vrms of both left and right channels with the loads listed.

Note: The formula for output power is V squared / R = P. P = Watts, in our case milliWatts.

 

[mt877]

Just so everyone knows, this amp actually has higher output power. When I first tuned it I started with the 32Ω load and went up to 1kΩ, then I checked 16Ω afterwards.
With the 16Ω load I couldn't turn the volume to max (almost to max) without severe clipping, so I took a measurement of the 16 just before clipping occurred. I was thinking about leaving the tuning I set with the 32Ω, but I settled on retuning starting with the 16Ω which reduced the power.

In reality, nobody would ever feed in consumer line level audio (1Vrms) and turn the volume to max with 16Ω head gear. You would blow your ears out. I could see feeding line level and turning the volume up to a comfortable listening level which would not even get near the potential signal clipping range. The other reason I settled with the 16Ω tuning is because I listened to some tunes with the higher output tuning and it sounded too clinical, edgy, no fun. Lack of a better way to describe it, the lower power tuning had a better overall balance. Just sounded better to me.

I may adjust the gain to high output again just for the headroom and give another listen. There's no way I would crank the volume to maximum if I was feeding line level into this amp.

EDIT: I went back and adjusted to the high gain. My initial impression with high gain adjustment was based on listening to 16Ω earbuds with my DAP at full volume input to the "Koala Amp". The high gain is beneficial to driving higher impedance head gear which is why I built this amp in the first place. For lower impedance head gear I just lower the volume of my DAP and turn the volume knob up on the Koala Amp and everything sounds great now.

Here's the higher output numbers:

Load resistor	Left Vrms	Right Vrms	Output Power per channel (V x V)/R = P (in mW)
15.7Ω (Closest value to 16Ω)	2.627	2.594	439.56 (Before signal clipping)
33.5Ω (Closest value to 32Ω)	3.605	3.605	387.9
82Ω (Closest value to 80Ω)	3.750	3.764	171.49
150Ω	3.797	3.794	96.11
300Ω	3.820	3.823	48.64
580Ω (Closest value to 600Ω)	3.829	3.823	25.28
1000Ω	3.830	3.824	14.63
Output Impedance = 1Ω

Procedure:
Input: 1khz sine wave @ 1Vrms (2.83Vpp)
Connect left and right 33.5Ω (32Ω) loads
Adjust internal gain pots for equal (matching) maximum sine wave output without clipping.
Measure Vrms of both left and right channels with the loads listed.

Note: The formula for output power is V squared / R = P. P = Watts, in our case milliWatts.
I adjusted the gain while viewing the sine wave on a scope and made sure to have nice rounded peaks of the sine wave. I could have turned the gain pots to max., but the sine wave was kind of pointy instead of rounded. Hope that makes sense.