(AK) classA HA/Preamplifier.

[jonclancy]

Hi Will,

Oh yes!!! This amp seems to suit the K701. Though it needs more volco than the stunt-cans I was testing.

Koss Porta Pros show their (understandable) limitations. TheTechnics RP-F800 stunt-cans are reasonable, if not actually quite good. I'll try the Superlux cans later on.

I'm currently cooking and checking. As mentioned, I'm running 400mA bias the the front end is not drifting too much at all since I put the cover on. I was a bit mistaken as it's your PSUs that I am running (specced to 35V) with double height heat sinks.

I got a bit serious and dug out a couple of the reference CDs I use (American Beauty and Sera Una Noche). Not tried Sera yet, but AB was, frankly, stunning through the K701. I'm tempted to try it out as a preamp, but will hold back as the main system is in disarray at the moment. As I have a modular build, I need to just mod my #2 pair of modules with SF12 and the output resistor, and swap them out. It's a screwdriver job.

Future jobs include changeover to DC umbilicals (regs and trafos in separate box, JLHRE in the main box).

My twin white LEDs cause a hum when installed on just one PSU, so much have a play with that. It looks nice to see the perspex disc white lit (and goes with the look of my power amps - when I get the switch wired in and momentary latch on/off board completed) - but I'm not that fussed. I removed the LEDs off the psu boards because I had a faff with them.

Here are some pics:

Open:



Cooking:



Ransacked??? I think I should clear this lot up!!



Cheers

Jon

Off to check the front end again...

[Deleted]

Jon,

Looking good there

My curiosity however, was diverted firstly to the coffee mug, forcing me to make one immediately, you and your subliminal images

Thence to the space under the desk, is that a Marantz receiver I spy?

[jonclancy]

It's a bit like one of those "seach for something" books.

It's a retro Marantz tuner. Lovely condition, mid-fi but the lights look very good!!

That's a huge mug - does nearly a pint of (red bush) tea, I reckon!

[jonclancy]

BTW, what would the max sensible bias be?

I'm running 160mA at the moment, and I could propably hold my finger on the sink for a minute without blubbing*.














*Not tried this in case it hurts.

[Deleted]

Feb 3, 2013 18:53:59 GMT jonclancy said:

BTW, what would the max sensible bias be?

I'm running 160mA at the moment, and I could propably hold my finger on the sink for a minute without blubbing*.


*Not tried this in case it hurts."

Hi Jon
I would suggest 200mA, although I haven't noticed much improvement going much above 100mA. Any additional heat as a result of much higher bias could possibly affect the tracking of the front end balancing without improved case ventilation, or at the least make that adjustment harder to get right..
Regards
Alex

[Deleted]

''Oh yes!!! This amp seems to suit the K701''

Hi John

Yes my take also
I remember how far my jaw dropped when I heard this HA with the 701. Just lovely.
My 701’s sound sensational through Alex’s Class A HA. It just really brings them to life in a way that other amps seem to struggle to match.

701 have no bass

Ha what a laugh

They have really good deep bass through this amp as I’m sure that you have noticed along with great overall detail and clarity.
But the sound stays musical. (And that’s not an easy trick to pull out of the hat)

Does the same trick with Alex’s Class A PA also when partnered with my PMC sound like a bigger version of the HA K701.i’m pretty sure that the combo would sound super good on your SD OB mmmm I’m almost jealous on that John.

I’m still loving my set up (as you can tell) and I’m glad that you’ve had the chance to hear for yourself.

With the 701 I find that I’m using the last ¼ of the pot setting which is plenty loud and proud and gives enough variation in volume for my needs.

Just one thing more

With Alex’s PA the HA/PRE is ghostly quiet through my speakers(90DB TL’S) but with my much more sensitive audiosector dual mono chip amp (much more sensitive on the IP) it is just a little more noisy.(only to be expected)

Just so you know to be gentle with the vol co if you plug it into your MYREFC first time out.

Great to hear that you’ve crossed the finishing line almost.

and really nice build

Take care

[jonclancy]

Cheers Guys. I decided to leave it on all night and the case to is pleasantly warm. I'm going to do the finger test on the sinks and a last check of the bias and front end. If there is headroom, I might just go to 200mA bias to see what it's like.

Yes Shaun, the K701 do have BASS!!

[jonclancy]

Yup - blubbing after finger held onto the heatsink for more than 5 seconds. I think I'll stick with 160mA bias for now.

I couldn't wait and tried the Class A as a pre. Remembering Shaun's advice, I turned the volco right down. Glad I did!!!

Only 4 notches for a huge sound! If I was using this as a dedicated pre, I'd reduce the gain!

Sounded great, but I know I have a little ground hum from my MyRefC set up - the trafo attenuator doesn't show this, but the Lightspeed clone and Class A (to a lesser extent) do. A job to get around to.

But, the amp is now back upstairs and playing some Sera as I contemplate the metalwork (on a plain black steel box) for the trafos and reg boards. This I'll just have to whip up a single JLHRE to get a set of 4 while I look to trying out Greg's excellent dual implementation.

[Deleted]

Feb 4, 2013 20:20:11 GMT jonclancy said:

Yup - blubbing after finger held onto the heatsink for more than 5 seconds. I think I'll stick with 160mA bias for now.

I couldn't wait and tried the Class A as a pre. Remembering Shaun's advice, I turned the volco right down. Glad I did!!!

Only 4 notches for a huge sound! If I was using this as a dedicated pre, I'd reduce the gain!

Sounded great, but I know I have a little ground hum from my MyRefC set up - the trafo attenuator doesn't show this, but the Lightspeed clone and Class A (to a lesser extent) do. A job to get around to.

But, the amp is now back upstairs and playing some Sera as I contemplate the metalwork (on a plain black steel box) for the trafos and reg boards. This I'll just have to whip up a single JLHRE to get a set of 4 while I look to trying out Greg's excellent dual implementation.

Hi Jon
If you use this as a dedicated pre with the 15W Class A you won't need to reduce the gain if you followed the original gain setting values in the schematic..
Perhaps your existing power amplifier has much higher gain than the 15W Class A, as my attenuator is usually below 12 o'clock for a comfortable level from DTV and CD/DVD.
Regards
Alex

[Deleted]

''Hi Jon
If you use this as a dedicated pre with the 15W Class A you won't need to reduce the gain if you followed the original gain setting values in the schematic..
Perhaps your existing power amplifier has much higher gain than the 15W Class A, as my attenuator is usually below 12 o'clock for a comfortable level from DTV and CD/DVD.
Regards
Alex''

Hi John

Alex is spot on and if my Audiosector Chip amp is anything to go by the input sensitivity is laaaarge and much more suited to a passive/buffer or TVC(something without gain). i'd not go for fiddling with the gain tooo much as the PA is so much better than the Audiosector.

Now with Alex's Class A PA it's just a perfect match and likewise I never get my Vol co much past 12 oc.

I hope you plan on building the PA as the Pre works really well driving it.

take care

[jonclancy]

Yup- I am!

[Deleted]

Feb 5, 2013 22:46:22 GMT jonclancy said:

Yup- I am!

Hi ERRR Jon (got the name right this time)

good stuff

i'm sure you are going to love it and will make a nice match for the OB's .

take care

[jonclancy]

Hi Shaun,

I didn't notice!!! You're forgiven. That's the trouble with evening surfing without a backlit keyboard - and hence the short answer!!

Yes, the MyRefCs are using a TVC and go as loud as I could ever want them to. It's a different side of the same (decent sound) coin. I haven't used an active pre for a long, long time. Even my Arcam D290Ps were happy with a passive set up. What's interesting is this is the first non-op-amp headphone amp I have built. I was looking at some of the Gilmore designs a few years ago, but the complexity and cost put me off.

The front end adjustment is fiddly - not that it drifts, but my trimmers seem to need a "twist past then back" approach to set the front end. IIRC, there have been small comparator boards knocking about that would do this job. Or am I imagining it??

The cheapy black box I'm using for the main power looks promising after a dry lay-out. I'll need to stack the trafos, so will be looking for some long screws from eBay (nylon if I can find them). In the meantime, I have my dims, so it'll be a case (hurrr) of masking, marking drilling and punching ready for the off.

Oops Part 2: Didn't post this yesterday...

I'm ordering a couple of Bulgin switched, fused IEC inlets to make the job a bit easier.

cpc.farnell.com/bulgin/bzh01-z0000-10/iec-inlet-dp-switched-fused/dp/CN01733

CPC are offering free delivery on web orders until tomorrow, so I thought I'd jump in on that.

[Deleted]

Hi John

i'm not using my Audiosector at the moment but liked it also with Glasshouse C core TVC (not as nice as your MF ones but pretty good) with it and it does seem to make a good match SQ wise.

My system will be moving this year to an upcoming music room above the garage we plan to have built (waiting on planning permission). So my wife got me thinking on a compact music system for the house. I’m planning a TP Volumite/ Opus/Ventus HA/pre into the Audiosector and a pair of speakers to be decided on later.

the HA/PRE Is a little fiddly to set for the first time but I found that leaving it switched on and giving plenty of time to warm up helps. Long crock clip leads left attached so that you can keep the lid on whilst warming also a big plus (as per Alex’s instructions)

Then once she settles down at say +12mv (random number but you get my drift (pun intended)) offset. Then take the lid off and once the offset finishes moving to say +24mv. Then just adjust the offset back to +12mv. When the lid goes back on warm her up again and repeat.

That should get you somewhere near.
Repeat a few times and you are good to go.

Trying to set with the lid is a PITA.

Hope that makes some sense errr Jon

Be prepared to be amazed when you have Alex’s class A up and running (but you’ve had a preview already if I remember rightly) so you know how good it is.

Take care

[Deleted]

Hi Alex,
I've a problem.....................thankfully not...........PSA
with the positive JLH section, at switch on the voltage rises normally from zero to full 20 volts then blows the recently installed 500ma fuse.
The positive  PSU Reg section  is working normally as is the negative side including its JLH section.

This is the Combo PSU which drew a high current and cooked the C/L toshies. 
I replaced the 2Sa & 2SC and their emitter resistors with 1.4R and installed fuses as links between the Reg section and JLH, powered up
only to find negative section good and positive side JLH faulty as above, any suggestions as to what might cure the fault.
All traces are clean so no obvious S/C to see.
Regards,
Alan

 

[Deleted]

Feb 9, 2013 0:39:12 GMT drymdrum said:

Hi Alex,
I've a problem.....................thankfully not...........PSA
with the positive JLH section, at switch on the voltage rises normally from zero to full 20 volts then blows the recently installed 500ma fuse.
The positive  PSU Reg section  is working normally as is the negative side including its JLH section.

This is the Combo PSU which drew a high current and cooked the C/L toshies. 
I replaced the 2Sa & 2SC and their emitter resistors with 1.4R and installed fuses as links between the Reg section and JLH, powered up
only to find negative section good and positive side JLH faulty as above, any suggestions as to what might cure the fault.
All traces are clean so no obvious S/C to see.
Regards,
Alan

 

Hi Alan
Are you able to test that JLH with a bench supply, or with a dummy load such as a few parallel PW5 resistors?
We need to know whether it's the JLH or the amp's PCB.
Is the front end of that Class A module's +VE rail definitely isolated from the +VE supply rail of the Output section ?
Regards
Alex

[Deleted]

Hi Alex,
just up no brekky yet, must be the JLH positive section only which is faulty as it blows the positive side fuse as soon as it's voltage rises to 20v, you see the voltage rise slowly as normal on power up unconnected to any load. Whilst the negative side is still operational with a steady -20v.
Must have a cuppa and a bit toast.
Thanks,
Alan

[Deleted]

Feb 9, 2013 9:47:35 GMT drymdrum said:

Hi Alex,
just up no brekky yet, must be the JLH positive section only which is faulty as it blows the positive side fuse as soon as it's voltage rises to 20v, you see the voltage rise slowly as normal on power up unconnected to any load. Whilst the negative side is still operational with a steady -20v.
Must have a cuppa and a bit toast.
Thanks,
Alan

Hi Alan
I suggest you try it at a lower voltage such as 9V, and do the measurements as in the old fault finding chart.
The main reading is just under 1.8V WRT 0 volts , at the BC639 end of the 120K resistor. (3 x VBE in series)
The 2SC5171 with the 33 ohm emitter resistor may have been damaged previously ?
If it went s/c it could possibly take the other 2SC5171 with it, depending on whether he CL section was still working correctly.
Check for a s/c between +VE Out and the top of the 33 ohm resistor (emitter of the 2SC5171)
Kind Regards
Alex
P.S.
The schematic shown is a superseded version.

[jonclancy]

Hi Guys,

Here's a little update:

I've marked out the back and bottom PSU case panels for centre-drilling and hole punching later on in the week. CPC were speedy in delivering my IEC inlets, so there is nothing to stop this project from being 90% completed in the correct configuration in a week or so.

Jobs to do will be to mod board set #2 with SF12 and output resistor value after consulting here, and knocking up another single board JLHRE for the temporary install (prior to dropping in some Black Boards).

Marking out:



General layout:



There will be two of those 80VA trafos stacked. Why so over-rated? Well, 80VA was only £2 more per trafo than 15 or 30 VA IIRC - seemed sensible.

[Deleted]

Hi Jon

Coming along nicely.

I have the same 80VA x 2 transformers in my build for the same price reason. Nothing wrong with a little over rating IMHO all runs nice and cool.

With the SF12 in place the front end balance is much less jumpy and sounds a little better also so well worth the effort.

Great work and better looking than my frogs in a box effort.

Take care

[gommer]

It's official now, Will kindly agreed on sharing the artwork for a new round of PCB's. Purely non profit and exclusive to the RockGrotto. Anyone wanting to obtain some new PCB's (1.6mm) for this project, should look here:
rockgrotto.proboards.com/post/126950/thread

Cheers, Marc

[Deleted]

Hi Alex,
right hand side of #2 build seems to be working, offset has settled at -00.3mv with an opa2134 installed, with AD744 it was 00.00 to 00.01.

I'm inclined to think that my misplaced 100uf on the 79L plus the first OPA I used being duff was the only problem on the HA/Pre board..............hope so anyway.
With the first OPA  the offset with it in was around 368mv and out 6mv   and that OPA got a little hot, the replacement OPA has been in now for around an hour
with a steady -00.3mv. I've still to tack the 2n2 in place..............that'll probably feck it up again. 
Bias is set at 175mv with comfortably warm heatsinks, I'll put it up to 200mv when I case it up.
The LTP has bottomed out nicely balanced so that looks good to set when cased.

I'm thinking that the S/C which blew up the combo PSU was contained within the PSU.
I replaced all the components on the positive side  of the JLH section and it worked for about 30 secs then blew the fusees on both sides.

I junked that board cut 2 more built one up and installed it as a working module  running since late afternoon today
so it looks good to go for #2 build.
Next up two AK balanced phono modules to build, this time I'm thinking on using  LM317/33  Regs to drop the voltage
taken from the JLH out on the combo PSU only as I've a good few in stock otherwise I've LT1086 in hand but no LT1033.

Regards,

Alan 

[Deleted]

[Deleted]

Hi Alan
I presume you mean an OPA134 ? For the previous offset corrector to get warm there had to a fault situation, as the load is through a total of 85Kohms.
Even a couple of mV DC out after the offset corrector is fine. The main thing is that the opamp is low noise in this particular application.
I am still in awe of your PCB making skills without using chemicals !
The LM317/LM337 wil have very little work to do with a JLH ahead of them,. so the noise from them will be quite low.
Kind Regards
Alex

[Deleted]