[shaneotool]

I don't really know what's going on these last few pages with the mod discussion on these amps, but I have a stock one - 12AU7 version - and it sounds great.

I dont have a soldering iron and don't plan on doing anything to it - just tought I'd throw this post in there in case someone reading this thread thinks you have to do all kind of work to make it sound good...

 

[schawo]

Quote:

Originally Posted by shaneotool /img/forum/go_quote.gif I don't really know what's going on these last few pages with the mod discussion on these amps, but I have a stock one - 12AU7 version - and it sounds great. I dont have a soldering iron and don't plan on doing anything to it - just tought I'd throw this post in there in case someone reading this thread thinks you have to do all kind of work to make it sound good...

It sounds good, but you can make it sound great.

 

[Avro_Arrow]

Quote:

Originally Posted by oohms /img/forum/go_quote.gif From my time playing with the amp, i'll list the mods i've done in order of most improvement to least 1. Change mosfet to IRF510 or similar 2. Buy more tubes for it and test them out 3. Change gate resistors from 220 ohms to 120 or so (only if you have upgraded the mosfet) 4. Do the crosstalk mod (pic below - only for the 12au7 version) 5. Experiment with output resistor values (stock: 47 ohms, try 68 ohms) 6. Swap passive components for better quality ones Here is the crosstalk improvement mod - it ONLY works with the 12au7 version! Cut the traces marked in red and connect the 2 points where the blue 'wire' is Taken from: Rock Grotto Audio Forum - For Headphones - Headphone Amps - Amplifiers - X-Can V2 - Musical Fidelity - headphone Discussion - Amplifier Discussion - DIY - Amplifier Kits - Projects - SCHA - Sennheiser - Beyer - Grado - Audio Technica - Headphone amp

If you do the Bias Cap mod you will have a new #1 mod...

I have IRF510 in mine, I'll try dropping the gate resistor to 150R and see how it sounds. Technically, you don't always need a gate resistor, but it enhances stability in most cases. Maybe I'll try with no gate resistor and see if it stays stable...

 

[Avro_Arrow]

Quote:

Originally Posted by Avro_Arrow /img/forum/go_quote.gif If you do the Bias Cap mod you will have a new #1 mod... I have IRF510 in mine, I'll try dropping the gate resistor to 150R and see how it sounds. Technically, you don't always need a gate resistor, but it enhances stability in most cases. Maybe I'll try with no gate resistor and see if it stays stable...

I just had a look at the 12AU7 thread you linked to and I see they just connected the cathode directly to ground and don't use any bias...the cap on the input is there because there will be quite a bit of current wanting to flow out of the grid...they really screwed up the design even if it does sound good...

 

[Judge Buff]

I have a totally new sounding amp tonight.

First I replaced my inconsistent switch with this one from Mouser. I had to splay the legs a little to get it to fit in the holes, but it wasn't a hassle. Feels quite solid and its specs say 30K repetitions.

I've used 16V 470uF Silmic II output caps for months with no probs and great sound. I wanted to go back to 1000uF caps and I decided on Nichicon ES Bi-Polar (Muse) at 16V. Their diameter just fits and I've been breaking them in all evening. I like the KW that A_A recommends (I have a 10K one as my PS reservoir.), but some folks swear by Muses like others swear by Black Gates. I had to try them...

I also diverged from A_A on my bias caps a little. I went with the Nichicon R7 Functional Polymers, which are new at Mouser, in a 1000uF. I really don't know if size makes a actual difference here, but A_A explained them as reservoirs and I like big reservoirs.

After I conduct bond hearings in the morning, I'm going to the Shack for the MOSFETS! More to come...

 

[Avro_Arrow]

Quote:

Originally Posted by Judge Buff /img/forum/go_quote.gif I have a totally new sounding amp tonight. First I replaced my inconsistent switch with this one from Mouser. I had to splay the legs a little to get it to fit in the holes, but it wasn't a hassle. Feels quite solid and its specs say 30K repetitions. I've used 16V 470uF Silmic II output caps for months with no probs and great sound. I wanted to go back to 1000uF caps and I decided on Nichicon ES Bi-Polar (Muse) at 16V. Their diameter just fits and I've been breaking them in all evening. I like the KW that A_A recommends (I have a 10K one as my PS reservoir.), but some folks swear by Muses like others swear by Black Gates. I had to try them... I also diverged from A_A on my bias caps a little. I went with the Nichicon R7 Functional Polymers, which are new at Mouser, in a 1000uF. I really don't know if size makes a actual difference here, but A_A explained them as reservoirs and I like big reservoirs. After I conduct bond hearings in the morning, I'm going to the Shack for the MOSFETS! More to come...

I changed by Bias Pots and MOSFETs yesterday...much easier now to set my bias. The IRF510 made a big difference in the top end, more than I suspected it would. It really brought the top end forward. Bells have more "tinkle", snares have more "snap". I even ended up turn my BBE enhancement off.
I guess I will have to burn it in for a few more days before I get the final sound for this mod...

A note for people doing these mods...
If you are having trouble with the tracks lifting off the board you might be using too much heat. I set my iron for 500 degrees and I didn't have any tracks lift.

 

[Judge Buff]

I got my IRF510s this morning, but I'm holding off on changing-out the MOSFETS until I feel a need to change them. This little amp sounds so fine right now with a CBS rebranded Siemens ECC88 in it.

Crystalline highs, smooth mids and deep bass; expansive stage with a wonderful 3D feel where the instruments "move" up, down, forward, back... I'm truly just in awe of the electronic gestalt I'm experiencing this afternoon! Lecky is another head-fier that uses the term "synergy" in the same manner that I use it. What I have together right now needs no other tweaking at this moment.

Tomorrow, it may be different, but for now, it's perfect and you don't screw around with perfection...

 

[xtlee]

One thing I noticed is that this amp has a lot of gain. The true output power is not shown in the specifications. Anyone know what this amp puts out in wattage?

 

[schawo]

According to the specs the amount of gain is 30dB. That is about 1000mW without knowing the target impedance.

 

[xtlee]

Quote:

Originally Posted by Avro_Arrow /img/forum/go_quote.gif It would be hard for me to rank them in order as they are all worthwhile... The most important would have to be adding the Bias Capacitors, they are missing from the original design. The Power/Output Caps and MOSFET will all get you noticeable improvements. Replacing the Bias Trim Pots is more to replace a poor quality component with one more suitable. If you plan on doing much tube rolling, it is much easier and accurate to adjust. Finally, if it still has the poor quality push button on/off switch, just remove it, jumper where it was and turn the whole thing on and off with a power bar. If it has the newer toggle switch then it should be OK. Here are some component suggestions, they are by no means the only ones that will work but are strongly suggested. I included Mouser Part Numbers. Power Bypass Cap: one needed 1uF @ 63 volts : 505-MKS21.0/63/10 (replaces the cap directly behind the power jack)

What about the other little ceramic cap between the on/off switch ad the mosfet? What's that one used for and is it the same value as the other one?

I grabbed the schematic and I don't see those 2 components in it.

 

[Avro_Arrow]

Quote:

Originally Posted by xtlee /img/forum/go_quote.gif What about the other little ceramic cap between the on/off switch ad the mosfet? What's that one used for and is it the same value as the other one? I grabbed the schematic and I don't see those 2 components in it.

The Ceramic cap directly behind the power jack is the power HF bypass cap. You can change this one to a 1uF film cap if you like...a Wima MKS2 is a good choice here. The other ceramic cap beside the power switch is for arc suppression when you turn the switch on and off. It prolongs the life of the switch. The only problem is the stock switch is underrated and poor quality. You would do better to change the switch than bother with that cap. I just removed my switch and replaced it with a jumper. I turn everything off with a power bar anyway.

 

[Avro_Arrow]

Quote:

Originally Posted by xtlee /img/forum/go_quote.gif One thing I noticed is that this amp has a lot of gain. The true output power is not shown in the specifications. Anyone know what this amp puts out in wattage?

I always love this question...
The theoretical maximum power output for this amp is about 3.5 watts. In reality it is going to be much lower. Probably more like 1 watt.
If you have a true RMS volt meter you can figure it out for yourself...
Just drive the amp with a steady 1kHz test tone and measure the AC volts at the negative side of the output cap. You can either have your headphones connected or a dummy load for exact measurements. Lets say you measured 10 volts AC (true RMS). Now lets say you were using Grado cans at 32 ohms. Now add that to the 47 ohm resistor in the output path for 79 ohms. Divide your output voltage (10 volts) by your output load (79 ohms) and you get .126 mA of current. Thats pretty close to the max .150mA of maximum current available. Now multiply the output current by the output voltage.
You get about 1.26 watts. Looks like the Amp was designed for 80 ohm loads and can produce about 1.5 watts maximum. If you are using higher impedance cans like AKG then your power will be less because you will run out of voltage before you get close to the current limit. If you have lower impedance cans like my 16 ohm ear buds you will hit the current limit and your power will be less as well. Remember also that the resistor in the output path functions as a voltage divider as well so your 32 ohm cans will get a little less than half the actual output power....

 

[pj_rage]

Quote:

Originally Posted by Avro_Arrow /img/forum/go_quote.gif I always love this question... The theoretical maximum power output for this amp is about 3.5 watts. In reality it is going to be much lower. Probably more like 1 watt. If you have a true RMS volt meter you can figure it out for yourself... Just drive the amp with a steady 1kHz test tone and measure the AC volts at the negative side of the output cap. You can either have your headphones connected or a dummy load for exact measurements. Lets say you measured 10 volts AC (true RMS). Now lets say you were using Grado cans at 32 ohms. Now add that to the 47 ohm resistor in the output path for 79 ohms. Divide your output voltage (10 volts) by your output load (79 ohms) and you get .126 mA of current. Thats pretty close to the max .150mA of maximum current available. Now multiply the output current by the output voltage. You get about 1.26 watts. Looks like the Amp was designed for 80 ohm loads and can produce about 1.5 watts maximum. If you are using higher impedance cans like AKG then your power will be less because you will run out of voltage before you get close to the current limit. If you have lower impedance cans like my 16 ohm ear buds you will hit the current limit and your power will be less as well. Remember also that the resistor in the output path functions as a voltage divider as well so your 32 ohm cans will get a little less than half the actual output power....

Why do you "run out of voltage" with higher impedance cans?

Are you basically saying that since Power = Voltage^2 / Resistance (impedance), if V (max) is constant, then as R (load) increases, the output power is less and would need more V to maintain the same power? I'm guessing this is what you mean by "running out of voltage"? Maybe the next logical question would be, how hard would it be to modify the unit for higher voltage, and therefore more power to higher impedance loads?

 

[Avro_Arrow]

Quote:

Originally Posted by pj_rage /img/forum/go_quote.gif Why do you "run out of voltage" with higher impedance cans? Are you basically saying that since Power = Voltage^2 / Resistance (impedance), if V (max) is constant, then as R (load) increases, the output power is less and would need more V to maintain the same power? I'm guessing this is what you mean by "running out of voltage"? Maybe the next logical question would be, how hard would it be to modify the unit for higher voltage, and therefore more power to higher impedance loads?

yes, that is what I am saying...I try to keep things in simple terms most people can relate to...

Well, to run at higher voltage...The tube can run at up to 300 volts or so if my memory is correct, the MOSFET is good up to 100 volts or so depending on which one you have (I changed mine to IRF510 which is good to 100 volts). Oh course you will would have to change your electrolytics to reflect the higher voltages too and you would need bigger heat sinks...maybe it would just be easier to buy a different amp...

 

[xtlee]

Out of curiosity, has anyone tried a PCC88/7DJ8 tube in these amps? Apparently, the only difference is in the heater requiring 7 volts. That's a 0.7 volt difference from the regular 6DJ8 tube. There are claims that most circuits will accommodate the tube and work just fine. Like here:
 
http://www.tubedepot.com/nos-7dj8.html