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Old 26th January 2003, 12:28 PM   #21
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John

Please don't be cruel. I'll be very interested in your bridge amp providing the 110v rms out. Even without component values.
Regarding the oscillator, fine tuning a (lamp) Wien bridge produced less than 0.01% second harmonic which should be more than sufficient especially compared to 5-6% off the wall. I use an 8015 AVR with an EPROM based lookup table and DAC at the output port. Mostly for the convenience of a second phase being readily available at another port. So far i've only built step-up transformer based amps to produce the high output but since comparing this to a Valhalla decided that high voltage amp is likely a much better way to go. The Valhalla produces much better subjective dynamics and bass. I did draw a circuit off the Valhalla board hi voltage amp and disappointingly it is cap coupled to the motor. A dc coupled amp will presumably offer better performance but at the risk of frying the motor if anything goes wrong. I wonder how Lingo deals with that.
Any info?


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Old 26th January 2003, 12:39 PM   #22
dhaen is offline dhaen  Europe
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Default Tease over.

Peter,

The bridge amp produced up to 240 volts out. (120v each side).
It is a dangerous beast to get working.
It's imperfect, in that there is a slight crossover glitch between the top and bottom FET's, but if I remember rightly, it practically disappears when on load, with the correct power-factor correction.

If you are determined, send me a disclaimer and non-disclosure e-mail. Since you are a valve man, I'll assume that you know what you're doing with these lethal voltages, and will isolate accordingly.
I won't send a PCB though - that would just be too tempting

Cheers,
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Old 11th March 2003, 10:35 AM   #23
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Default Implementation help

Dhaen,

Thank you very much for your interest, the pcb offer and the info. I've spent time to understand the garrard circuit, and the one you have is far above my head of electronic knowledge today. With the pcb you have, I only have to add the power stage, right? Would be it expensive in components?

Peter & Brett,

Thanks a lot for your information and scheme. I think I will build the scheme, but have some comments:

I don't think the thorens motor will need much power, but don't know the actual value.
I've found (with the 166) that with 220V the motor vibrates and make noises, and that in the 160 there is a resistor for lower the voltage. Putting a similar value solves the noise problem of the motor, so I'm sure that the motor is the same, it is for 110V operation, so I only have to reach this value in the ps.

Oscillator:
what is the minimum value for the trafo? is it necessary 50VA really?
What is the purpose of the 5k pot?
The non-linear element (the lamp) is just a 28V lamp?
The pulley I have is for 60Hz operation, so it has a smaller diameter. I've been thinking about it and maybe I keep it because the lesser the contact the lesser the vibration passed to the platter, right? Is because of this that the top-models has a very small pulley and just a wire instead of belt?
The values I will consider for 60Hz operation are:
R: 270 K
C: 0.01uF
I will use the 5532, because I already have one.
I like your suggestion about to put in a cap after the oscillator and put out the electrolitycs in front of the trafo. I don't know wich value should I choose, any help??

Power Amp

If the purpose is to amplify the signal, here I can use any amp capable of manage a load equal to the trafo impedance and with the highest voltage swing possible, right? Any reason for the 3886? The distorsion of the circuit is given by the oscillator, by the amp, or by the trafo?
The 100K pot at the possitive input of the LM is a "volume" pot, isn't it?

Why could be better use a high voltage amp instead of a trafo?

Step-up Transformer

Can I simply use an ordinary trafo, but backwards? Yep, it's so simple...

Oh boy, toooo many questions today, sorry and many thanks for your help. I'm real analog, (I bought last week more than 100 very good lp's for 140 EURO) so why to buy a CD?

Thanks a lot
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Old 11th March 2003, 02:40 PM   #24
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The Thorens motor will work with 90v rms if you remove the resistor.
As i don't have a 160 anymore i'd suggest you have a look at the stated power consumption at the back of the unit and use a transformer with at least double that. Surely 20VA will be more than enough.
The 5k pot sets the feedback for oscillation. A scope can be very handy here as you need to monitor how the oscillation grows. Set it for around 2v p-p and a clean looking sine. In the abscence of a scope you can use one of the many scope/analyser progs which work with a soundcard. Because of the low frequency you'll be able to very accurately measure the harmonics in the output (that's optional)
28v 20-30mA seems to work very well. i used the first lamp i found and it was fine. the scope/ spectrum analyser will confirm this.
With a 100k input resistance the coupling cap value is non-critical. Just 1uF preferably non-polar will do.
You don't have to use 3886. I used briefly a Rotel power amp and a PP EL34 amp. The 3886 is damn convenient though.
The bulk of the distortion is caused by the step-up transformer. If you have one specially wound you may choose to go for more turns/volt than the min requirement for the core. It seems this will minimise the distortion. I ended up using a 50VA 220-20 v unit. At 90v the distortion is very low.
Yeah, the 100k is volume. Use it to set the output.

Good luck

peter
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Old 11th March 2003, 04:02 PM   #25
Raka is offline Raka  Europe
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Default Thanks

I have downloaded the LM3886 data, and you were right: it's pretty convenient to build a simple amp as long as it is going to deal with one frequency. My only concern is if the load (the trafo) is going to be an easy load. A standard trafo has not 8 not even 4 ohms, is this a problem. I was just comparing to an audio amplifier feeding just a wire (like a trafo), wouldn't that increase the distortion??
Now it's time to make a PCB.

Thanks a lot for your support, it's greatly appreciated. I think you deserve your nickname.
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Old 11th March 2003, 04:25 PM   #26
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You're welcome.
The transformer represents a very low load only at DC - you may be talking less that 1ohm for the primary. This is only a problem if there is any significant DC at output - 3mV in this case - so nothing to really worry about. At 50Hz (60) where you're really developing power the LM sees the impeadance of the motor stepped down by the square of transformer ratio - whatever is the impedance of the motor divided by about 100.
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Old 12th March 2003, 10:57 AM   #27
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Thanks for the info on the trafo impedance, now my concerns are away.

The thorens motor is only 1.5VA, but I'll oversize the power amp, just in case I upgrade my deck or my motor someday. Depending on transformers price, I will settle on 30-50VA.
What I'm not sure yet is if I should include a speed change. I don't have 45 records, but who knows!! I don't think changing the speed very often would be a good idea with this circuit, but as I will have two standard pulleys (one for 50Hz, and the original one for 60Hz), maybe I should fit the two options and i will have the chance to buy some 60Hz motors in the future... Or just leave space to fit the new RC values for different pulleys-motors, ... just thinking.
I have made the pcb layout, but for the NE5532 (dual), I will make it more neat and fit a 5534.
I know seems the same question as always but, what about the heatsinking requirements for the LM?
Just for the record, I've ordered the RB250 with the expressimo heavyweight and OL VTA. I hope that all this will result in a good deck to listen to my Prokofieff records.
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Old 12th March 2003, 11:23 AM   #28
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Hi

What do you need a pcb for? A project as simple as this will take you an hour max to wire using vero board. No idea what the actual power dissipated in the Thorens will be, but with the Garrard the LM gets red hot with a respectable heatsink. No big deal to make it switchable for 45rpm, dunno if you'll find much Prokofiev stuff on 45 though

cheers

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Old 13th March 2003, 10:44 AM   #29
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Yes, it's a simple project, but I always spent a time thinkink about the layout even for a veroboard. Besides, my target is now to build a good power supply even for future projects. I'm trying to design a layout that allows for switchable RC constants, maybe a small "card" that could be plug to the board with this small conectors (I don't know the name, but are used for jumpers). You know, 60HZ? well, let's plug this card, now a bigger pulley? well, let's fit this one...
About the heatsinking, when you say respectable, with size are you refering to? 200*100mm?
My "spagetti" layouts fed me up, so that's why I now spend more time thinking than building. I'm becoming the saint Job.
BTW, yesterday I measured the mains distortion and was only 2%(at 19:30). Do you really think a "perfect" sinusoudal is going to improve the sonics? I'm going to build the ps so you will have then my comments.
Another thing (maybe related) I discovered is that the motor is noisy even stopped. I used a "stetoscopio" and a bzzz is heard when the motor is just conected. That bzzzz reaches the arm. When the motor is started and in normal operation, the bzzzz rises, but not much.
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Old 13th March 2003, 12:28 PM   #30
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Raka,

Quote:
With the pcb you have, I only have to add the power stage, right? Would be it expensive in components?
Yes, it's the power stage you want. Really an amplifier of the required power feeding into a step-up transformer would be the easiest.
You need not use the original strobe circuit. That is complicated by the need of the original to be mains isolated.

I still have 3 boards left, so you can have one if you'll use it.

Cheers,
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