Symasym 5.3 "AAK model" builder's thread - Page 22 - diyAudio
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Old 25th July 2007, 06:31 PM   #211
anatech is offline anatech  Canada
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Hi Andrew,
Quote:
old fashioned analogue multimeters were great for this.
They certainly are.

Hi MBK,
Can you change the sampling rate on your DVM. Some meters will allow you to drop resolution to trade for speed. Your DC offset would have to be much higher to do what you have reported.

Why not make an RF detector probe for your meter? All it will tell you really is whether you do in fact have oscillation or not. Also, check out the temperature of your zobel resistor. It may get warm (finger tip test).

-Chris
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Old 26th July 2007, 03:45 AM   #212
MBK is offline MBK  Singapore
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Hi Chris,

Quote:
Originally posted by anatech


Can you change the sampling rate on your DVM.

Will look into this. It's a Craftsman though, not a Fluke (I am aware of that other thread!) . One time I think I saw a 2V and once a 13V number flashing by (DC offset), for fractions of a second, that would sound more like the woofer movement, next reading around 50 mV, slowly decaying to 0 - 3 mV, all this in 1 - 3 seconds.

Quote:
Originally posted by anatech


Why not make an RF detector probe for your meter?
Now that would be great, how do I go about this? I dimly remember seeing an article using a Germanium diode I believe. I sometimes use the child monitor (basically a walkie talkie receiver) as a primitive detector but it is too sensitive, the hash from the nearby transformer bridge is enough to get you plenty noise there (!!).

Zobel, when I had the amps on a bench I often checked, never warm (even with the floating heatsink problems). Now mounted in the box direct access to the PCB is a lot harder to impossible unfortunately.
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Old 26th July 2007, 08:47 PM   #213
anatech is offline anatech  Canada
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Hi MBK,
Quote:
I dimly remember seeing an article using a Germanium diode I believe.
That's the idea. A silicon diode followed by a small capacitor and a resistor to ground. Measure across the resistor on your DC scale.

For measuring your start up DC offset, can you lock your meter to the 20V scale? This will save time in autoranging. Still, integrating a changing signal will give you a very inaccurate answer.

I am totally spoiled with my HP 34401A. They still sell for $$$$$$$ on Ebay used!

An inexpensive pointer type meter may be your best friend right now. No good for high impedance circuits, but fine on an amplifier output. You can try it with your made up RF probe. Try and "Google" the phrase.

-Chris
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Old 27th July 2007, 02:27 PM   #214
MBK is offline MBK  Singapore
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Hi Chris,

This HP meter looks superb indeed. Well, they also have plenty of old oscilloscopes 2nd hand in store here, around 200 USD for the 20 MHz models, w/o probe. I am always on the verge of breaking down for one, I just don't really trust what they mean with "working condition" and I don't want another riddle to solve: is it the scope that's not working or is it me that doesn't know how to use it? haha. not.

For sure I saw old style analog multimeters in the stores, new, for cheap, so that part is easy. RF probe: so a small signal Silicon diode will also work? Thought it needed to be Germanium.

BTW I saw in another thread you have trouble finding M3 taping screws and 2.5 mm bits over there. Well I bought my taps, box of 3, Japanese, for around 2 USD here, and the 2.5mm bit (Black and Decker, they do know metric!) for another, err, 1 USD or so. PM me if interested, this stuff doesn't weigh much and should stand in for the beer I offered earlier.

To Ryssen, AAK, Cromodora and all others: do you also get a turn on thump? or is it just me?
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Old 27th July 2007, 02:38 PM   #215
Ryssen is offline Ryssen  Sweden
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No real "thumb",when connect the mains,I get a hum,not to loud,wich slowly faids away 1-3 seconds,until it is "quiet".
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Old 27th July 2007, 02:55 PM   #216
MBK is offline MBK  Singapore
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OK that reassures me, sounds like what I am getting, except on a 96 dB/W woofer it is kinda audible.
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Old 27th July 2007, 03:11 PM   #217
AAK is offline AAK  United States
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Hi MBK,

I get a barely audible thump on power up. I've never tried using the MPSA18 for an input, but, it's possible using the 2N5551 instead could be causing it. The MPSA18 has a much higher Hfe.

Al
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Old 27th July 2007, 03:23 PM   #218
MBK is offline MBK  Singapore
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Thanks Al, well my batch of 2N5551 measured a meager 100-130 Hfe, same as the drivers and output devices actually.
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Old 27th July 2007, 04:22 PM   #219
anatech is offline anatech  Canada
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Hi MBK,
The low hFE could cause this. Your gains do seen like they are very low. You are in a very good area to pick up Japanese semi's (real ones). 2SA970 and 2SC2240 are favorite devices of mine. Very, very popular in amplifiers. If you redo the pinout, they would work just fine in there. They are also good to 120 V C-E.

Can you get large quantities for <$?> ? Look at 100 lots in a BL gain ranking. The data sheets are PDF's over our posting limit. PM me and I'll send them to you.

Magura may send me a 3M tap and drill (2.5 mm). However, I sometimes do a lot of work. I break things if I'm too tired and too stubborn to go to sleep. If you can check some of these out for me I should pay you. Do you have a PayPal account?

There may be some other Japanese part numbers that would suit this project well. I'm thinking driver transistors. Other favorites of mine are 2SC3421 and 2SA1358, 2SC3423 and 2SA1360, 2SC3467 and 2SA1370. There are more. These are old classics and I have to find new TO-220 drivers. On Semi has been making some new outputs that are finally well worth using. Let's hope they flesh out their line to cover drivers and smaller signal devices.

I see some really good deals on 'scopes on Ebay in your area. For $200 you should be able to pick up a 100 MHz scope. Understand that they will all need work by now. Don't be tempted to buy some of the inexpensive digital 'scopes. With a DSO you need a ton more performance to make it as useful as an analog 20 MHz 'scope. That means $$$$$$$$.

-Chris
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Old 27th July 2007, 07:51 PM   #220
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Quote:
Originally posted by MBK


To Ryssen, AAK, Cromodora and all others: do you also get a turn on thump? or is it just me?
Sorry. Have not sorted amp out yet.

Current status:
Yet to install new pair of BD139s. Brand new black ones from Farnell to replace grey surplus ones bought from you know where.

Also just finished rebuilding Carlos' AD815pre.
This and the Symasym will form a one box integrated. No nulling circuit. If I read that thread correctly, can do without as the Symasym has caps at input. Only setback is the input DC will eat up volume pot wipers.

Mulling over:
While the heatsink is unmounted, I'm thinking of changing the feedback cap to Oscon 470uF/10v. Also, while at it why not I change R5 and R6 from generic to better types. These degen resistors greatly determine noise level of input pair right?
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