Just received a couple of MF X-A50 mono amps, one of which doesn't work.

The seller tried them both before sending them to me.

Does someone know a good repairer to send it to?

My retired serviceman friend couldn't recommend anyone down here for the job.

Thanks.

Remind me, where are you located?

DarkLantern blog - darklanternforowen.wordpress.com

Hastings Owen.

I've been recommended only Wgtn repairers anywhere near you I'm afraid - see thread, WELLINGTON repairers.
Others may know someone more local.

They appear to be relatively simple amps.

DarkLantern blog - darklanternforowen.wordpress.com

Have you done the usual checks first, check fuse, making sure light on front panel is on etc?

AMR-iFi R&D

Hi Colin.No fuse easily accessible and the power light lights up.
Downloaded a circuit diagram from HiFi Engine and there's no fuse shown in it!

Thanks Owen,will look into Wellington repairers.
Anyone have Marcus Wilson's phone no?

Hi Neilsan, just checked the schematic, like you say there is no fuse! Just a thermal fuse in the power transformer so that if it overheats it’ll blow. A little disturbing really.
If the front panel light is on the circuit will be receiving power. I’d check to see if there is any dc on the output. If there’s just the usual less than 100mV suspect the input socket or wiring.
Edit: I just remembered they use fuses in the power plug here in the UK. Which makes operating this amp without a fuse in the power plug in NZ dangerous and possibly a fire hazard. If you’d like me to get you a coupe of UK power plugs and use them with UK to NZ adapters let me know!

AMR-iFi R&D

Thanks Colin,being an electrician you know how to get me worried!!!!!
I spoke to Marcus today and he's keen to have at it so I've sent the 2 monos to him.
I'll check out fusing when they return.
Thanks again all.

haha MF build quality hasn't improved since The Preamp days

😬Don’t mean to get you worried, just trying to help with an issue to be safe.
Anyone else running UK equipment, please ascertain that it has a fuse or other safety feature for protection!

AMR-iFi R&D

Marcus rang last night to say he'd found the fault.The package must have been dropped and the back plate bent,which I did notice.Anyhow it had broken off the input cable,so after pulling it apart,which I didn't want to do,he straightened the panel,reconnected the cable,and he loves the bass they put out for 50w.(I sent both amps).

He agrees with colinf that I should fit a fuse in the line somewhere,so thanks again colinf and others who suggested Marcus.
I had a job recently changing a lot of english plugs to NZ ones, so kept a few of the fuses in them but not the plugs.Pain!

Ok, good to hear it was just the input wiring.
If you want a UK plug with 3A fuse in it Element14 Australia have it, cat. no. 166 1469. Or a power cord, cat. no 246 0366. You can use that with a UK to NZ plug adapter. That might be the easiest way to add a fuse. Otherwise you could add one with an inline fuse holder inside the amp behind the IEC socket.
😊

AMR-iFi R&D

Or change the IEC socket to a Fused IEC socket?

DarkLantern blog - darklanternforowen.wordpress.com

Thanks colin and Owen.The only trouble with fitting a fused IEC socket in the X-Can is you have to slide the circuit board out of the can,and the transistors are fastened to the inside of the can with all that goo,which is why I didn't want to open them myself.

I've thought of fitting a small metal box in the lead to hold a fuse, but I think colin's idea of getting a couple of UK plugs will be safer. I've had some caps from Element 14 for the Haflers so will contact them.

I've had the top off the X-P200 amp and can fit a fuse holder inside that OK.

Cheers! Have you bought them to listen to something other than the Hafler?

AMR-iFi R&D

x-series was uber-cool, loved those extruded cans.

Oct 23, 2019 19:54:01 GMT 12 colinf said:

Cheers! Have you bought them to listen to something other than the Hafler?

I bought an MF X-P200 a few months ago and love the dynamic bass and clean treble off it,not as thick sounding bass as my 2150B or the Haflers.

The Haflers...I've replaced all electro caps, some resistors, bias trimpots and a few other mods in one of them.The other one is still standard and mint. Modded one sounds musical,easy listening,no hassles,but the X-P200 is more detailed and lively. P200 very revealing of which preamp I use,which is why I sold the Plinius M7 after trying it out!

So when the X-A50s with X-Pre and X-Can2 were listed I was interested.

After a struggle I decided not to bid on them,but a friend had given me some money and in the end I decided to buy now!

Going to try using the 50s on my tweeters and the P200 on the bass drivers,seeing they're all matched for input. ( Image 412/2's)

For fusing the A50s I'm making up a double switch socket on an enclosed mounting block,with a panel mount fuse holder in the side for each socket, with a lead out to wall socket.

Nice one, enjoy!
Biamping could be a good idea, is the gain on both MFs the same?

AMR-iFi R&D

Hoping so! I read somewhere that all MF's had matched gain,so you could biamp.
Will find out soon.
Just a bit concerned about the "passive" Clone Note into 2 lots of amps.Can't find anywhere what the MF's input impedance is.

Looking at the X-A50 schematic, the input impedance is 25k decreasing to 15k at 20kHz. There is an RF filter at the input made up of a 2.2k resistor in series with a 330pF cap. That combo might roll off the top end from a passive preamp, depending on its output impedance. Driving 2 MFs with the same filter at the input might sound dull. I suspect these amps have been designed to be driven from an active line stage.
What’s the output impedance of the LDRs in your passive volume control? To drive 2 MFs you’d probably want a resistance no more than 10k.

AMR-iFi R&D

Thanks colinf. I didn't know how to work that out.

My Clone Note's LDR's are adjusted to 6K,but can be adjusted up to 10k.
Uriah Daley recommends 6k so I haven't tried any other setting.
Guess I'll find out when the amps get back!

Oct 26, 2019 18:30:14 GMT 12 colinf said:

What’s the output impedance of the LDRs in your passive volume control?

That’s something I’ve often wondered.

They look to be essentially a voltage divider, a resistor attenuator with 1 LDR in series, 1 LDR in parallel to Gnd.

So, output Z should be low at low volumes, but how much does this impedance vary with increasing volume?

DarkLantern blog - darklanternforowen.wordpress.com

The Clone Note I'm using is the same setup as the shunt type attenuator on the bottom of this page, with the LDR in place of the In-Out resistor.

Also Uriah uses solid state relays in place of a switch to switch the shunt resistors to ground.

I also have one of Goldpoints series attenuators.Prefer the CN.

www.goldpt.com/attenuator_types.html

Yup, the downstream amp 'sees' 2 Rs, 1 in series, 1 in parallel to the source device.

But colinf would be better to explain the imped specif of the LDRs.

DarkLantern blog - darklanternforowen.wordpress.com

Have you got a circuit diagram of the Clone Note?
So in operation, you select a shunt resistor to ground to set the volume coarsely, and use the LDR to set the volume finely?
The output impedance of a 10k pot at its highest output resistance (about 2 o’clock position on a logarithmic pot) would be 2.5k, assuming the source impedance is low. The output impedance of a 6k Clone Note at half volume would be 3k. But since it’s a shunt type volume control, the output impedance would increase, approaching 6k, assuming the LDR is still set to 6k. In practice for full volume you’d set the LDR at its least resistance, about 1.5k.
So that would be fine to drive 2 MFs.

AMR-iFi R&D

I'll have a look colin.I run the setup off a 12v battery.

I adjust the LDRs to 6k, well one of them drifts a wee bit,so check them occasionally with a multimeter.So primary/adjustment side of the LDR is fed from the battery,but audio in-out is isolated from power. (Other side of LDR,adjusted to 6K.)


The volume pot just turns on/off the solid state shunt relays/resistors,( optomos) depending on where it's positioned.

The LDR audio in-out side could be replaced with a resistor.

Should try that on my other one!!

Back in 2017, we had a discussion on the basics of volume controls, including LDRs - in an early thread here.

DarkLantern blog - darklanternforowen.wordpress.com

Thanks Owen.
Solid state relays, nice.

AMR-iFi R&D

Got the X-A50's back from Marcus.It was a loose wire caused by the courier/packaging damaging the back plate.
Sound marvelous,bit fuller bass than the X-P200, but very similar elsewhere.
Then yesterday I noticed the repaired mono getting very hot,so it's back to Marcus they go!

Oh...
If that’s the case I’d replace the 1k bias pot (PR1 on the schematic) as it might have drifted with time. Just resetting the bias with the original pot might allow the bias to drift again later.
That leads to the next question...what to set the output bias current at? It’s a quasi-complementary (QC) output stage and crossover distortion when it’s set to class B operation is high. Class AB is better but suffers from asymmetrical transconductance doubling in a QC output stage. Class A is by far the best but leads to very high dissipation in the output transistors. I suspect the XA-50 has been designed with class AB operation in mind, looking at the heatsink arrangement. So I’d set it about 100mA (50mA per output transistors pair). Of course if you can handle a warm-running amp higher bias is better. As long as the bias transistor TR7 is well thermally coupled to the heatsink as well.

AMR-iFi R&D