[JimL11]

Hello everyone! I recently entered the electrostatic world and bought an SRM-T1 and an SR Lambda signature. I love DIY projects, so I specifically got the T1 with the intention of replacing the caps, putting in safety resistors, and doing the CCS mod that's been discussed earlier in the forum. I have the older T1 with the caps running across the amp rather than down the side. As far as I can tell, there's no pics of the CCS mod in this version of the amp. So here's some pics! This was a lot of fun to put together, and the result is wonderful.

I used a combination of the 2017 magazine article from @JimL11, the German blog posts from @Firschi, and conversations between JimL11 and muffinhead at around page 923-934 to piece together how to do this. So big thanks to all of you for documenting this. It made my life a lot easier.

Glad you liked it! The basic Stax circuit is an excellent one, as evidenced by the fact that it is still used in the SRM-006 and SRM-007 more than 30 years since the original T1 was released for sale. I specifically designed this as a low cost, high value modification. The parts cost is less than $100, of which more than half is the cost of replacing the electrolytic capacitors, which is routine maintenance in amps of this age.The CCS mod more than doubles the effective power of the amp by removing the signal power wastage in the output plate resistors, which removes a number of its sonic flaws while maintaining its basic character. This can be measured objectively by looking at the maximum voltage output of the modified amp (> 400 VRMS at 1 kHz), which is limited only by the power supply voltages, vs the stock amp (300 VRMS at 1 kHz).

 

[gasmonkey]

Forgive me for not reading 1200+ pages,

I own a Threshold S/150 II OB (90+ watts, about 20+ pure class A).

Wondering if I can just unhook my speakers and hook up Stax headphones? I figure if I already have lots of class A...... Why not? I use ROON, and control the volume digitally from my phone/laptop, so I can set the volume as low as needed.

Im assuming there are better ways to go about this, but I cant help but get my brain stuck in a loop when thinking about a $600 energizer VS. a top notch piece of class A amp I already own.

Even if I could do this, is there an advantage to using a Stax specific energizer over a class A power amp?

Thanks for getting me started.

 

[walakalulu]

Are you having a laugh? Energisers and class A amps are diferent kettles of fish. Good way to blow up your headphone though...

 

[Jon L]

Forgive me for not reading 1200+ pages,

I own a Threshold S/150 II OB (90+ watts, about 20+ pure class A).

Wondering if I can just unhook my speakers and hook up Stax headphones? I figure if I already have lots of class A...... Why not? I use ROON, and control the volume digitally from my phone/laptop, so I can set the volume as low as needed.

Im assuming there are better ways to go about this, but I cant help but get my brain stuck in a loop when thinking about a $600 energizer VS. a top notch piece of class A amp I already own.

Even if I could do this, is there an advantage to using a Stax specific energizer over a class A power amp?

Thanks for getting me started.

You need to buy either Stax SRD-7 Pro or SRD-7 Mk II transformer box, which you connect to your Threshold amp's speaker outputs to power Stax 'phones (modern "pro" models). Your Threshold will probably sound quite good with them

 

[DaddyWhale]

You need to buy either Stax SRD-7 Pro or SRD-7 Mk II transformer box, which you connect to your Threshold amp's speaker outputs to power Stax 'phones (modern "pro" models). Your Threshold will probably sound quite good with them

Woo Audio (https://wooaudio.com/) and Mjolnir Audio (https://mjolnir-audio.com/) also make transformer boxes. Mjolnir's use transformers scrapped from Stax boxes with improved circuitry. I don't use either. So cannot directly attest to their performance. But Birgir Gudjonsson, who operates Mjolnir, makes excellent stuff. I own a Mjolnir KGSSHV Carbon amplifier (not a transformer) and could not be happier. Birgir is a (dedicated and opinionated) gentleman and, I'm sure, will answer any questions you might have about his stuff. I have no experience with Woo's technical merits. But they have been excellent to deal with: I bought my sr-009S from them. Excellent customer service.

 

[VRacer-111]

Forgive me for not reading 1200+ pages,

I own a Threshold S/150 II OB (90+ watts, about 20+ pure class A).

Wondering if I can just unhook my speakers and hook up Stax headphones? I figure if I already have lots of class A...... Why not? I use ROON, and control the volume digitally from my phone/laptop, so I can set the volume as low as needed.

Im assuming there are better ways to go about this, but I cant help but get my brain stuck in a loop when thinking about a $600 energizer VS. a top notch piece of class A amp I already own.

Even if I could do this, is there an advantage to using a Stax specific energizer over a class A power amp?

Thanks for getting me started.

No advantage of a STAX unit over a stereo amp + electrostatic transformer needed to use estats with a stereo amp. I ran mine with NAD C275BEE stereo amp and Mjolnir Audio modded SRD-7 transformer. Gives better subbass presence and fuller sound compared to the STAX SRM-323S I first had. Very satisfying sounding with a stereo amp. If I weren't getting out of estats I'd be keeping the Mjolnir SRD-7: it does very well with good quality amps.

Since amps really are more important for the sound of estats than with planar / dynamic headphones, the amp you use will really impact the sound characteristics. Every estat system I've heard sounds different... the amp gives it's characteristics as does the DAC. Amps, DACs, and EQ or anything else in the chain can be used to tweak the sound. If you have an amp you like the sound of, the estat will show quite a bit of that amps characteristics.

 

[JimL11]

Forgive me for not reading 1200+ pages,

I own a Threshold S/150 II OB (90+ watts, about 20+ pure class A).

Wondering if I can just unhook my speakers and hook up Stax headphones? I figure if I already have lots of class A...... Why not? I use ROON, and control the volume digitally from my phone/laptop, so I can set the volume as low as needed.

Im assuming there are better ways to go about this, but I cant help but get my brain stuck in a loop when thinking about a $600 energizer VS. a top notch piece of class A amp I already own.

Even if I could do this, is there an advantage to using a Stax specific energizer over a class A power amp?

Thanks for getting me started.

To elaborate, there are two reasons you cannot just connect a speaker amp like the Threshold to electrostatic headphones:

1) electrostatic headphones require a bias voltage of either +230V (standard bias Stax) or +580V (pro bias Stax). This is supplied with electrostatic headphone amps or Stax converter boxes, Although the Woo converter box also does this, there have been issues with Woo using non-standard bias safety resistors or no safety resistor, as documented by spritzer, who is also the person behind Mjolnir Audio. Note that the earlier Stax converter boxes only supply the standard bias. All current Stax headphones use pro bias so only the SRD-7Pro or SRD-7 II will work correctly (one way to check is the pro bias sockets accept 5 pin jacks only whereas the normal bias sockets accept 6 pin sockets. If you hook up a speaker amp without supplying the bias voltage you will get no sound at all.

2) electrostatic headphones require very little current but high voltages (i.e. 100 volts or more for loud levels). Speaker amps are good at supplying lots of current at relatively low voltages, which is why they require a converter box (which has two step-up transformers for the signal voltage and the aforementioned bias circuit). If you supply the bias voltage and hook up a speaker amp directly to the electrostatic headphones you will quickly hear the amp clipping due to insufficient voltage swing.

Note that the same thing is true of electrostatic speakers. All electrostatic speakers which do not have a built in amp have built in step-up transformers which increase the signal voltage of a standard speaker amp up to several THOUSAND volts. They also have a bias supply, which is why they all have to be plugged into the AC wall socket.

 

[Tugbars]

Forgive me for not reading 1200+ pages,

I own a Threshold S/150 II OB (90+ watts, about 20+ pure class A).

Wondering if I can just unhook my speakers and hook up Stax headphones? I figure if I already have lots of class A...... Why not? I use ROON, and control the volume digitally from my phone/laptop, so I can set the volume as low as needed.

Im assuming there are better ways to go about this, but I cant help but get my brain stuck in a loop when thinking about a $600 energizer VS. a top notch piece of class A amp I already own.

Even if I could do this, is there an advantage to using a Stax specific energizer over a class A power amp?

Thanks for getting me started.

Hello,

I'll just dump everything I know about this topic using your question as an excuse. I might be wrong at some points, I'd be glad if someone can correct me.

Direct drive amp vs Speaker Amp + Trafo:

There are 2 factors electrostatic headphones require to have full 20-20khz voltage swing: Enough voltage(determines how high the volume can get on your headphones) and current. Without enough current the stators can't get filled with electrostatic charge in the given time, the voltage sags, treble extension starts failing. Current output of an amp affects bass performance too.

Theoretically, a powerful class A amp + Stax trafo can give all the voltage & current electrostatic headphones can need. This is the MOST efficient solution to drive electrostatic headphones. Few direct drive amps like Carbon & T2 have enough current output to supply full voltage swing they offer. On the other hand, you can drive any electrostatic headphones to their full potential by using a 300$ vintage amp & 400$ stax SRD7-mkII too. So why should someone bother buying direct drive amps then?

The problem with using trafos is that the capacitance caused by high winding rate of trafos cause bad treble extension and bad treble performance in general. To my experience speaker(I've tried pass labs x250.8 too) + Stax trafo solutions always have some grain and lack of sparkle in treble region. Lundahl transformers solve this issue to an extent. Lundahl transformers have a special low capacitance winding technique which allows them to provide better treble extension. Moreover, since lundahl transformers can provide up to 50mA current(Mjolnir Carbon has 20mA current output for example), the dynamics can be a bit even better than direct drive amps. For this purpose Mjolnir audio used custom made lundahl trafos which have 1:19 winding rate(stax trafos have 1:27 winding rate if i can recall correct) I'd strongly recommend you to get a lundahl trafo box if you don't want to make compromises on how trebles will sound on your system.

In short, Lundahl trafos have far superior treble extension & bass performance & dynamics compared to what Stax trafos can offer. It's important to mention here that powerful direct drive amps are expensive but they offer totally uncompromised treble performance. Lundahl trafos & a good speaker amp comes very close and even maybe exceed them in some areas.

Those lundahls have very low efficiency so make sure to use a powerful amp. Good luck in your search.

 

[jsts]

Regarding CCS modification.

Once @JimL11 wrote that it remains a mystery why Stax does not use CCS-mod in its amplifiers, although it improves efficiency. Thinking about this, I believe that one of the reasons is the “metallic” sound of transistors installed after vacuum tubes instead of resistors. Don't you think so?

 

[Mach3]

Regarding CCS modification.

Once @JimL11 wrote that it remains a mystery why Stax does not use CCS-mod in its amplifiers, although it improves efficiency. Thinking about this, I believe that one of the reasons is the “metallic” sound of transistors installed after vacuum tubes instead of resistors. Don't you think so?

If that was true why on earth does the T2 uses CCS too? Correct me if I'm wrong.

 

[Mach3]

Direct drive amp vs Speaker Amp + Trafo:

Those lundahls have very low efficiency so make sure to use a powerful amp. Good luck in your search.

Thanks Tugbars for the info, really appreciate it.
Not sure if my finding can add to yours.

I have a pair of Stax ELS-F81 electrostatic speaker that has a trafo built in.
Stax built a mono block that was design to be paired up with the F81 model Stax DA-100M. Class A 100 watts into 8 Ohm.
However they die off their asses at high volume. This is due to the F81 weird impedance curve. 4~512ohm 512 Ohm at 20hz or 4 Ohm at 20khz. Yes ouch to solid state amps
sensitivity was a low 73db/1w/1m
I was recommended to use a tube amp instead to over come this issue. Recommended by Kent from ElectroStatic Solutions.
I'm currently using a Primaluna 100 watt tube amp.
I was wonder if a tube speaker amp + Trafo would be even better than a solid state speaker amp.
Anyone tried this?

 

[JimL11]

Regarding CCS modification.

Once @JimL11 wrote that it remains a mystery why Stax does not use CCS-mod in its amplifiers, although it improves efficiency. Thinking about this, I believe that one of the reasons is the “metallic” sound of transistors installed after vacuum tubes instead of resistors. Don't you think so?

Very doubtful. The T1, 006 and 007 all have transistor front ends and intermediate stages, so why wouldn't that contribute more transistor sound than the CCS. And the CCS are not "after" the tubes, they are in the same stage - the output stage, which is the only stage that has vacuum tubes.

And it's not as if they don't know about CCS loads, since their transistor output amps have had CCS loading since at least the SRM-313 (1999) and SRM-717 (2000). Also, the dual MOSFET CCS circuit is pretty transparent - it has an effective impedance of at least 150 MEGOHMS, so is very close to an open circuit and contributes very little to the sound. In fact, the SRX Plus circuit uses the same CCS loads but is otherwise pure tube and nobody who has listened to it has ever commented that it has "transistor" sound.

If that was true why on earth does the T2 uses CCS too? Correct me if I'm wrong.

Yes, the T2 uses CCS, and it also has intermediate transistor stages.

 

[jsts]

Very doubtful. The T1, 006 and 007 all have transistor front ends and intermediate stages, so why wouldn't that contribute more transistor sound than the CCS. And the CCS are not "after" the tubes, they are in the same stage - the output stage, which is the only stage that has vacuum tubes.

Yes, the T2 uses CCS, and it also has intermediate transistor stages.

JimL11, you are as always great Thank you for the clarification. It's just that I am considering modifying my SRM-600 and I am afraid of losing the "valve" sound.

 

[Tugbars]

Thanks Tugbars for the info, really appreciate it.
Not sure if my finding can add to yours.

I have a pair of Stax ELS-F81 electrostatic speaker that has a trafo built in.
Stax built a mono block that was design to be paired up with the F81 model Stax DA-100M. Class A 100 watts into 8 Ohm.
However they die off their asses at high volume. This is due to the F81 weird impedance curve. 4~512ohm 512 Ohm at 20hz or 4 Ohm at 20khz. Yes ouch to solid state amps
sensitivity was a low 73db/1w/1m
I was recommended to use a tube amp instead to over come this issue. Recommended by Kent from ElectroStatic Solutions.
I'm currently using a Primaluna 100 watt tube amp.
I was wonder if a tube speaker amp + Trafo would be even better than a solid state speaker amp.
Anyone tried this?

You should use load invariant power amplifiers. Old threshold amps with Statis Topology, some of the Pass Labs amplifiers, Sunfire load invariant amp etc is what you need. There are also some tube amps which are load invariant, their prices are ridiculously high for me, So I didn't care to memorize them but you can see results with a quick google search I believe.

A friend of mine used Nakamichi PA7(has load invariant Statis topology) to drive an electrostatic system, apparently the amp makes everything sound very warm.(It was very detailed sounding amplifier though) And he didn't like it. So vintage amp + load invariant topology isn't the only absolute answer.

I have an slightly off-topic question to you, Do STAX 009BK also sound different(in a better way) to you than 009S as well. That was my experience and I'm curious if it just me who thinks like that? If you've listened to 009S, let me know.

 

[GarageBoy]

Very doubtful. The T1, 006 and 007 all have transistor front ends and intermediate stages, so why wouldn't that contribute more transistor sound than the CCS. And the CCS are not "after" the tubes, they are in the same stage - the output stage, which is the only stage that has vacuum tubes.

And it's not as if they don't know about CCS loads, since their transistor output amps have had CCS loading since at least the SRM-313 (1999) and SRM-717 (2000). Also, the dual MOSFET CCS circuit is pretty transparent - it has an effective impedance of at least 150 MEGOHMS, so is very close to an open circuit and contributes very little to the sound. In fact, the SRX Plus circuit uses the same CCS loads but is otherwise pure tube and nobody who has listened to it has ever commented that it has "transistor" sound.



Yes, the T2 uses CCS, and it also has intermediate transistor stages.

Some people like the sound of the Bottlehead Crack without the Speedball - somehow it gets attributed to the transistors