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Millett "Starving Student" hybrid amp - Page 453

post #6781 of 6853
Quote:
Originally Posted by Kaaf View Post
 

I have a strange issue

When I'm using my Senn HD600 I don't receive any humm, But when I connect my new NAD viso HP50 I receive an annoying humm in my right ear. Not audiable when playing music but very annoying during silence. It isn't affected by the volumeknob

The HD600 has an impendace of 300ohm and the NAD has a impedance of 25 ohm. Could this cause this rather high pitched hum? And does somebody knows if it's caused by something in particular (already swapped tubes, problem remains right)

definitely the impedance of the headphones not being high enough for the output impedance on the amp... people had the same problem using low impedance cans with Schiit's Vali.

post #6782 of 6853
Quote:
Originally Posted by AHorseNamedJeff View Post

definitely the impedance of the headphones not being high enough for the output impedance on the amp... people had the same problem using low impedance cans with Schiit's Vali.

Quite unlikely. I regularly use my Grado phones (32 ohms) with my SSMH and I remember testing it with my Etymotic ER-6i (16 ohms).

It's probably a grounding/shielding issue which, unfortunately, can be a headache to eliminate.

I can suggest you can start by trying your amp with a different source, different input cables, in a different room, and maybe even a different power supply.
post #6783 of 6853

Hi All,

I've been planning to build a PTP version for quite some time, been reading this (long) post for quite some time. Currently at page 249 (no spoilers please). Unfortunately some of the old posts no longer have linked images working so I may have missed a few things. 

This would be my first point to point so I wanted to make sure I'm following correctly. I drew this diagram from the schematics. 

Does this look correct or am I completely off in my understanding of how to do ptp?

I can't post images so here is a link: http://bayimg.com/aapNCaAFk

 

- The pot is a dual, did not have it in the components of the program I used. 

- Grounds would all go to a star grounding.

- I'd follow indications I found across the thread re. positioning of resistors/caps close to certain components. 

- I'd be using 12AU7 tubes

 

Thanks,

xmx

post #6784 of 6853
Quote:
Originally Posted by ximamp View Post
 

Hi All,

I've been planning to build a PTP version for quite some time, been reading this (long) post for quite some time. Currently at page 249 (no spoilers please). Unfortunately some of the old posts no longer have linked images working so I may have missed a few things. 

This would be my first point to point so I wanted to make sure I'm following correctly. I drew this diagram from the schematics. 

Does this look correct or am I completely off in my understanding of how to do ptp?

I can't post images so here is a link: http://bayimg.com/aapNCaAFk

 

- The pot is a dual, did not have it in the components of the program I used. 

- Grounds would all go to a star grounding.

- I'd follow indications I found across the thread re. positioning of resistors/caps close to certain components. 

- I'd be using 12AU7 tubes

 

Thanks,

xmx

C6 doesn't look right. It should go from the junction of R13, R1, R2, R7 and R8 to ground. In your image it seems it's 'bridging' the junctions of R13-R1-R2 and R7-R8.

 

Aside from that I can't see anything wrong... BUT I just took a quick look and did not confirm the MOSFET pinouts :)

 

cheers!

post #6785 of 6853

wow thanks for spotting that, I had messed out big time. 

 

http://bayimg.com/EapOLAAfK

 

I numbered the MOSFET, 1=G, 2=D, 3=S and made the connections clearer, another thing I need to watch carefully when building. 

 

Thanks again!

post #6786 of 6853

That looks fine. But don't take my word for it! Verify against the schematic (and MOSFET pinout) as you build.  Good luck!

post #6787 of 6853

Allright, first post on the forum!

 

Anyway, time to jump right in. This is some work in progress I'd like to share with you guys & ladies. I’d like to build a simple hybrid headphone amp and the SSMH is really ideal for my purpose. I'm having some fun adding minor tweaks to the SSMH's design to make it a bit more flexible for my build. It's basically one way to bring the "Starving Student" back to the SSMH. Since the 19J6 is a bit scarce, I'm currently using the 12AU7-modded SSMH-schematic from Dsavitsk and the_equalizer as a baseline. With the appropriate adjustments, the SSMH can actually run with E88CC/6DJ8's and 6J6's, next to the already existing 12AU7-version. In fact, the standard PSU can run any reasonable 6,3V or 12,6V preamp tube with a current between 0 and 350 mA.

 

As I’m not allowed to upload pictures within the first few posts, I’ll just have to explain per keyboard. If you want to follow, get a schematic of the 12AU7-style SSMH by the hand. :smile:

 

Basically I used the same schematic, with only a few minor tweaks. Say I want to use a couple of E88CC’s instead of the 12AU7’s. Both are a dual triode but have different pinout’s. The 12AU7 has 2 heaters (one for each halve, rated at 6,3V at 150mA) internally in series. The heater has an accessible center tap so the heater can be externally configured in series (12,6V/150mA) or parallel (6,3V/300mA). The E88CC actually has the exact same heaters (2 times 6,3V at 150mA) but are internally connected in parallel. This means that it only can run in 6,3V mode at 300mA.

 

The first problem is the lower heater voltage of the E88CC and the 6J6 being 6,3V. So I added a couple of LM317HV constant current sources (IC + one resistor) between the mosfet sources and the tube heaters (a bit like the Sijosae MHHA v2.0 or the Bravo Audio V2 but at 48V). This decouples the heater voltage from the mosfet source voltage. The heater can then be configured according to one's wishes (6,3V or 12,6V) independent of the mosfet bias voltage. This is akin to putting a beefy power resistor in series with the heater to get a higher voltage drop. You can for example put a 21 ohm resistor in series with the E88CC heater and it will be equivalent to a 12,6V heater at 0,3A. The difference with a CCS is that the power resistor will pass AC voltage variations while the CCS will actively block most of the AC voltage variations, the constraint being the constant current draw. Since the Bravo Audio V2 is a working amp, I assume this tweak will certainly work, provided you use the HV-rated version of the LM317.

 

Now here’s where things get experimental. The second problem is that the current in these lower voltage heaters is a lot higher than your typical 150mA of the 12AU7. 300mA and 450mA are typical values for the E88CC respectively 6J6. So one mosfet carries the full heater current. Some tube amps use a string of preamp heaters in series as a cathode resistor to bias the output tubes of a push pull stage. This means you can e.g. have a couple of E88CC heaters in series giving you a compound heater of 6,3V - 12,6V - 18,9V - 25,4V at the same current of 300mA. Each output tube then gets 150mA of this heater current. The same concept can be applied to the SSMH. You use one or more tube heaters in series and the current then splits up into the mosfets for the left and right output stage. This can be done by having the heater shared between the left and right channel. The PSU can now be dimensioned at half the current of the traditional SSMH.

 

As an example, for a cool running SSMH I could use the heaters of a single 12AU7 in series, which gives me 12,6V at 150mA. Then I split the current into 2 times 75mA with each LM317HV configured as a 75mA CCS-load for it’s respective mosfet source follower. This can still be enough for some 90mW or more (e.g. 53mArms into 32 ohm) into the more sensitive medium to low impedance headphones while simultaneously halving the heat production to 7.2W (48V * 0,15A) instead of 14.4W (48V * 0,3A). Of course, nothing stops you from running the heater in center tapped 6,3V mode at 300mA and push 150mA per mosfet as in the original SSMH, while still using only one tube.

 

The following should be feasable for the original Cisco 48V PSU:

  • Single 12AU7 at 150mA with shared heaters
  • Single or dual 12AU7 at 300mA with separated heaters - this one contains the 12AU7-mod by Dsavitsk & the_equalizer
  • Single or dual E88CC/6DJ8 at 300mA with shared heaters

 

And for a more powerful 48V PSU:

  • Single or dual E88CC/6DJ8 at 365mA with shared heaters
  • Dual 6J6 at 450 mA with shared heaters - possibility to get the authentic SSMH 19J6 tone at a very low tube price

 

The next logical step could be to DC-couple the mosfet to the tube plate. So on the schematic, remove C2,R2,R4 and C4,R8 and R10 and attach the resistors R3 and R9 directly to the tube plate and done. You can then play with the plate load resistors and the cathode bias resistors of the tube to find a good quiescent point of the tube while simultaneously biasing the mosfet for the most optimal voltage.

 

These small tweaks should actually have little impact on the sound quality (except for fooling around with the plate load and cathode bias with a DC-coupled mosfet) while bulletproofing the amp for first time builders. Provided you solder the current setting resistors of the CCS correctly, then too high a mosfet bias cannot burn out tube heaters.

 

A tone altering tweak could be to add a plate CCS to the tube. This would compromise the tube swing as any active CCS uses a bit of voltage to operate. So make it passive. We already have a mosfet source follower hanging up there. Split the plate load resistor into 2 equal resistors and add a capacitor from the mosfet source to the midpoint of the two resistors. The mosfet source follower is a unity gain stage so couples all the AC-variations back to the midpoint of the two resistors. The lower resistor sees a constant voltage drop and a constant current. This is called a bootstrapped current source and requires only one extra resistor and capacitor.

 

The schematics are still experimental but look promising. The DC-coupled version actually looks simpler than the 12AU7-modded SSMH. I’ll upload ‘em as soon as they are trustworthy for some fooling around on my breadboard. In the meantime I’ll be happy to answer any questions. And if this whole contraption doesn’t work: I’m GlowGale, pleased to meet you. And I like subtile first posts. :D

 

Greetings from a real starving student,

GlowGale

 

EDIT:

  • Had to remove the single 6J6-option due to the common cathode between the triodes

Edited by GlowGale - 10/11/14 at 8:51am
post #6788 of 6853

Welcome to Head-Fi and sorry about your wallet....

 

Great first post!

You seem to have done your homework.

We will be pleased to see how your project turns out.

post #6789 of 6853

Thanks for the warm welcome, Avro_Arrow.

 

Of course everything has it's upsides and downsides. The standard SSMH averages the tube characteristics because of the two tubes in parallel, so with a single tube you will have a greater unbalance. Depending on the type of the tube, this could mean that you need to have bias pots on the cathodes and rebias every couple of months or so. The big advantage is that you can buy a good single NOS tube. The tube halves should have better balance than between different tubes, or at least a repeatable unbalance. The tube halves also have a greater chance of similar aging because they are being used both at the same time (assuming a good amp design they were in). So this is where flexibility comes in. Don't want adjustment pots, run a dual tube amp. If you want to be social & leave the second tube for your fellow SSMH-builder and don't have any problem adjusting pots once in a while, then go for a single tube amp. For the same pile of 6J6's, you can build double the amount of SSMH-amplifiers. :smile:

 

By the looks of it, I think i'll be having a go at a DC-coupled single 12AU7-build for starters and if the shared heater thing works, progress to a single E88CC-build.

 

Greetings,

GG


Edited by GlowGale - 9/9/14 at 4:16pm
post #6790 of 6853
Quote:
Originally Posted by GlowGale View Post
 

Thanks for the warm welcome, Avro_Arrow.

 

Of course everything has it's upsides and downsides. The standard SSMH averages the tube characteristics because of the two tubes in parallel, so with a single tube you will have a greater unbalance. Depending on the type of the tube, this could mean that you need to have bias pots on the cathodes and rebias every couple of months or so. The big advantage is that you can buy a good single NOS tube. The tube halves should have better balance than between different tubes, or at least a repeatable unbalance. The tube halves also have a greater chance of similar aging because they are being used both at the same time (assuming a good amp design they were in). So this is where flexibility comes in. Don't want adjustment pots, run a dual tube amp. If you want to be social & leave the second tube for your fellow SSMH-builder and don't have any problem adjusting pots once in a while, then go for a single tube amp. For the same pile of 6J6's, you can build double the amount of SSMH-amplifiers. :smile:

 

By the looks of it, I think i'll be having a go at a DC-coupled single 12AU7-build for starters and if the shared heater thing works, progress to a single E88CC-build.

 

Greetings,

GG

welcome, and looking forward to your E88CC version!!!

post #6791 of 6853
Welcome!

I've been thinking about building another Starving Student (this time for my desk at work), so I'm excited to see how your project turns out! Good luck with it!
post #6792 of 6853

Thanks guys for the warm welcome!

 

I did some field testing today with two LM317 current sources driving a light bulb (2,7W at 6V, nominal impedance 13,3 ohm). This test basically proves the concept that you can feed a common load with a positive temperature coefficient from two parallel current sources. Since I love fooling around with circuits, they survived several direct shorts and open circuits across the light bulb without the PSU shutting down. I also noted the soft start effect of the light bulb slowly coming on so little to no inrush current and no hiccuping of the PSU trying to drive cold filaments.

 

For those wanting to recreate the experiment: abused ATX PSU running at 21,49V when loaded with the experiment. Cold light bulb impedance was 1,3 ohm. One 10 ohm current set resistor per LM317 so configured at 125mA per CCS and measured 127mA when running (resistor & LM317 tolerances). Voltage drop over the light bulb was 2,354V and voltage drop over the CCS'es was 19,1V. This gives me a warm resistance of 9,2 ohm.

 

Cheers,

GG


Edited by GlowGale - 9/10/14 at 1:00pm
post #6793 of 6853

I'm also very interested in seeing how your prototype turns out!

 

Cheers!

post #6794 of 6853

This design seems really cool and simple! Are there any specifications or power measurements? Lots of pages, I only dug through the first several but it I am interested.

post #6795 of 6853

Hello Tubehunter, welcome to Head-Fi!

 

I can only give some approximate theoretical values for the 12AU7 version because I'm still having a go at completely understanding the circuit. The output power of the SSMH is mainly defined by the mosfet load, which is the heater. Here it is configured at 12,6V and 150mA.

 

As a side note: These are the quiescent values so you get 1,89W of heat dissipation per tube and 5,31W of heat dissipation per mosfet, 14,4W in total. This whole thing pulls 300mA from the PSU so the standard PSU is a Cisco 48V at 350mA, accounting for some inrush current at startup.

 

Anyway, I hope my calculation method is right. Basically take the square root of the heater values which is 8,9Vrms and 106mArms so these are the theoretical limit values. To find the max. output power, simply take the minimum value of the following equations: P = 0,106² x R and P = 8,9² / R where R is your nominal headphone impedance. The actual output power is defined by your audio source voltage and the amp gain. Gain should be somewhere between 10 and 16 (20 and 24 db) I think. Amplifier input impedance of the 12AU7 version is at 100k.

 

Frequency response, THD analysis and more of the original 19J6 amplifier can be found by following the link in the opening post of this huge thread.

Anyway, the SSMH seems to have enough power to drive most of the standard headphones on the market to a decent loudness.

 

Cheers,

GG

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