[evil-zen]

Is a cmoy or a CHA 47 better for the 501s? I prefer a warmer sound to it. And how do I make the amp more powerful??

I might buy a ET 4p (with adaptor) in the future so it's best that the above amp sounds good with it too.

Thanks!

 

[tophu]

The cha47 was designed for the larger current requirements of grados, which is overkill for the k501. The cmoy can easily supply enough current for any reasonable volume. Both of these amps will sound somewhat thin in the bass due to the dip in bass response in the k501s

However, I just finished a DIY amp custom designed for the k501s.

The basic specs are:
--buf634-driven virtual ground
--opa134 for the gain (socketed, going to try opa627, opa227, and ada797) buffered by a buf634 in a multiloop topology
--gain opamp biased to class A by a cascode current source
--bass boost tuned to compensate for the frequency response curve of the k501 (to cancel out the dip in the bass response)

In my opinion, it far exceeds the sound from a basic cmoy amp due to the better bass response.

If you're interested I can post schematics, parts list, and pcb layout.

If you're going to use it with ety's, you'll want to put a switch on the bass boost.

 

[Nisbeth]

Tophu, I´m very interested in the schematic for your amp, since I´m also considering the 501. Could you please post everything here or mail it to me at Nisbeth@hotmail.com

TIA
/Uffe

 

[formulacarl]

Tophu,
I too am interested in the amp that you have described. It would be great if you could post the diagram as well as the parts used. Thanks in advance.
Carl

 

[tophu]

Okay, this is going to be a long post.

Here is the power supply schematic:

This schematic shows the bandwidth pin shorted to ground, but in the pcb layout this is optional (you would have to add a jumper) as there were statements made by others that this could lead to instability. Note that this power supply assumes a reasonably quiet DC supply. I use a lab quality adjustable supply borrowed from work.

Here is one of the channels:

I'll include the parts list later on.

Here is the layout diagram:

And here is the postscript file that can be used for the bottom copper layout.

The board was designed to fit into a Serpac A-27 case with no battery compartment (DC input jack only). The DC jack was put at the back between the two big caps directly over the virtual ground buffer. The front panel (from left to right) is the power switch with the led immediately above it, the pot, then the two input jacks (angled inwards to make enough room for the wires).

There are a number of jumper wires required for the ground and power rails. These are labelled on the layout diagram. There are a number of ground points provided on the layout, these are for the input, output, and pot grounds (on the panasonic pot the two grounds are side by side, so bend them together and use one wire to the board). Finally, holes are provided for the power rails and the LED and series resistor. For the power rails and the main ground jumper, I used the thickest wire I could get through the 1mm holes.

Parts list:
Resistors (I used yageo 1%, I'd like to try Vishay some time but Mouser's not as convenient as Digikey to Canadian locations):
4.75K x 5 (I used one for the LED, if you use a different LED, adjust this as appropriate)
100 x 4
845 x 2
10K x 2
2.47K x 2
100K x 2

Capacitors:
.47uF x 4 polypropylene (I used Panasonic P-series)
.1uF x 8 polypropylene (also P-series)
10uF x 4 35V electrolytic (Panasonic FC)
100uF x 2 35V electrolytic (Panasonic FC)
2200uf x 2 35V electrolytic (Panasonic NHG)

Active bits:
opa134 x 2 (dip format)
buf634 x 3 (dip format)
n-channel jfet x 4 (2N5457 or similar)

Misc:
3.5mm stereo jacks x 2, threaded
DC power jack, threaded, solder lugs
DC power plug (to match jack)
SPST power switch (or SPDT on/on and only use half), threaded, solder lugs
high intensity blue LED (5V 20mA)
panasonic stereo audio taper pot, threaded, metal bushing
knob to fit pot shaft

Everything but the volume knob (bought at Radio Shack) and the PCB etching stuff (Active Electronics) was purchased at Digikey because it's got much more reasonable shipping to Canada. I can give part numbers if you need them. Other than the blue LED, the other misc. stuff is all on Tangent's cmoy tutorial so if you're stuck head on over and take a peek.

If you build it, let me know what you think.

Tophu

PS. The power supply voltage can safely be reduced it it is easier. The limiting factor will be clipping. I wouldn't want to go below 9V supply, 9-20V is the range I would be looking in.

 

[gavinbirss]

Hello,

You guys must wisper when you talk about the BASS dip with K501's.

I have listened to a pair of HD600's before I listened to my K501's on my own headamp prototype. I liked the BASS on the HD600's but I am not dissapointed with the K 501's BASS on a similar headamp. (OPA627 + BUF634).

I use only a gain of 6.7, a 50K pot, a 1M to ground on input and 26V DC that is split and I am happy with the power output of this amp.

I did however have some severe noise problems when I tried the OPA637 and AD797 in the same circuit. The OPA637 sounded fine until you changed the volume to almost min value to min and also max volume. It seemed to be sensiitive to the extremes ends of the volume pot. The AD797 did exactly the same.

When the OPA627 are used they do not produce noise when the volume is at zero (min) or at max.

Please let me know what succsess you have with the AD797 and if you have encountered noise problems with the OPA637 and others that is related to the volume pot, gain or input impedance.

I appreciate any advise.

Gavin

 

[tophu]

The 637 is very unstable and is designed IIRC for gains of at least 10.

In my simulations I could not get a reasonable phase margin out of it so I don't really plan on using the 637.

I'll try out the 627 and 797 though and let you know what happens.

 

[Nisbeth]

Tophu, just one more question: I might have overlooked something, but the switch you mentioned for the bass-boost, where does that go?
Otherwise, thanks for the plans and the board-layout, this looks like a great project!!

/Uffe

 

[tophu]

If you want to put a switch on the bass boost, then you'll need a double pole (one pole for each channel) single throw on/off switch (or a double throw on/on with one side not hooked up).

Drill out the holes for R3 big enough so that you can put some hookup wire down beside the resistor pins and solder them in place at the same time as the resistor.

When those two pairs of wires are shorted together, it will cut out the bass boost. Away from the low frequencies the gain should be almost the same with the switch on or off.

 

[Nisbeth]

Thanks!

/Uffe

 

[jarthel]

I would strongly suggest Kevin Gilmore's amp. I have the K401 and I love the sound.

for project details, head over to headwize.com

jayel