Technics 1210 internal regulation mods

[Mike_New]

I promised myself that I would not buy into power supply discussions, as the likes of Paul are well able to provide what people need with a range of excellent options.
However if those few folks who have the experience and electronic knowledge to persue this, then I will contribute my ideas.

In my opinion the better way to handle the tranny problem is to obtain one which has two 18V-20V secondary windings and rated at say 5A
A suitable unit can be obtained from Jaycar in Oz for $55.00 and are rated at 8.8A no less!!. No doubt they are of Asian extraction so they should be available from outlets in the UK and elsewhere.

Paul correctly states that inrush current situations, can cause core saturation problems with toroidal trannys, as they do not have an air gap; 'Inrush Current Suppressors' can be used which limmit the current at switch-on no matter at what point on the sine wave.
These have a variety of characteristics in terms of current and restore time.

Also these current suppressors will help to reduce the initial secondary current through the rectifier diodes with large smoothing cap values such as 20,000uF-40,000uF, as the winding see an initial short circuit before the caps charge up to the working voltage.

Using the two 18V windings (approx 23VDC with a FW rectifier) Then one supply can be used to feed the motor Drive/Regulator circuit at 21.0V
The second can be used to provide the 21V to the other control circuits and be further regulated down to provide the 9V if required.

Shunt any RF by using ceramic and tantulum caps. as suggested in the reference circuits provided for the LM317.

In fact for a few extra dollars the better option would be to use 25V AC windings, which will give a much 'stiffer' regulated supply, as the rectified DC will be bit less than rt2 X Vac or in this case about 35V DC The max input Vdc of the LM317 is rated at 40V.

[UV101]

By the sounds of it, my transformer is woefully inadequate!

I think a much larger VA TX is in order along with a jump in uF and pre regulation within the external PSU. Will a larger TX deal with core saturation? A 25v 120va would provide a possible 4.8A with parallel secondary's or a 25v 160va would give 6.4A. Both are less than £20 here from RS. How much better during normal operation would this PSU be given a larger better transformer?

My regs are FET based with feedback and should be more than capable of delivering large current to demanding loads so should be quite good here given the size of the heatsink!!

One final thing. My 18v TX is giving me about 28v when rectified. I ususally work on 1.4 x TX voltage but it seem this is incorrect. Can anyone shed any light for me please. The reason I ask is that my caps are 35v and I think I will se more than 35v DC on them using a 25vac TX.

Again Thanks

[Clive]

One final thing. My 18v TX is giving me about 28v when rectified. I ususally work on 1.4 x TX voltage but it seem this is incorrect. Can anyone shed any light for me please. The reason I ask is that my caps are 35v and I think I will se more than 35v DC on them using a 25vac TX.

You have the voltage drop from the diodes to take into account but most of all you have the "regulation" of the TX to add on. Small TXs can have a regulation factor of 15% or more so if you have a light load on such a TX you can be adding close to 15% to your TX output volts. Possibly I've over-simplified a touch but this is near enough.

A higher rated TX will have better regulation, you can check this in the TX specs. A very high VA TX will give you much closer to 25V but possibly more like 27V AC or 34V DC after the diodes (depending on diode type).

[chrism]

Hi Ian,

My trafo supplies 17.5v AC and around 27.5v DC after rectification. This gives a figure of about 1.5 to 1.6. Hope that this helps.

You could be close to 40v DC using the 25v AC trafo.

Regards

Chris

[MartinT]

One final thing. My 18v TX is giving me about 28v when rectified. I ususally work on 1.4 x TX voltage but it seem this is incorrect. Can anyone shed any light for me please.

Hi Ian

Your unloaded DC should be sqrt(2) * 18 = 25.4V but that could be rated for, say, 230VAC input. If your input voltage is the more typical 240-250VAC then that would account for the 28V you're seeing.

[Mike_New]

By the sounds of it, my transformer is woefully inadequate!

I think a much larger VA TX is in order along with a jump in uF and pre regulation within the external PSU. Will a larger TX deal with core saturation? A 25v 120va would provide a possible 4.8A with parallel secondary's or a 25v 160va would give 6.4A. Both are less than £20 here from RS. How much better during normal operation would this PSU be given a larger better transformer?

My regs are FET based with feedback and should be more than capable of delivering large current to demanding loads so should be quite good here given the size of the heatsink!!

One final thing. My 18v TX is giving me about 28v when rectified. I ususally work on 1.4 x TX voltage but it seem this is incorrect. Can anyone shed any light for me please. The reason I ask is that my caps are 35v and I think I will se more than 35v DC on them using a 25vac TX.

Again Thanks

Hi Ian,
A larger Tranny will not in itself deal with the saturation problem, but it will be less severe since there will be a lot more 'iron' in the circuit relative to the amount of current you are drawing from the secondary.

The correct way to solve this problem in my judgement, is to use 'inrush current suppressors' which only cost a few cents. However you must allow adequate space around them as they get hot! Essentially their resistance is inversely proportional to their temp. which is not exactly intuitive!!
They have a switching current of 3-6 amps at various switching times.

It is true that if you use the 25V AC option then you will need 50V rated caps. on the raw voltage side.

[UV101]

ok, that makes sense however, as I'm not a fan of anything "in line" how does this sound as an option.

using a quality relay with mains rated contacts that can be controlled by a simple timer circuit. An "inrush current suppressor" (a specific device or could be a resistor maybe???) could be placed in series with the primary windings. Once the initial start up has taken place, it could be shorted out effectively bypassing it.

[Paul Hynes]

Hello Ian,

The transformer I use in the SR5 is rated at 160 VA but it is wound on an oversize core with a lower flux density than typical “off the shelf” transformers. The secondary voltage is 25 volts AC. The cores and windings are carefully manufactured from high quality materials. The way the transformer is wound is also a large part of the performance and these transformers are manually wound on a toroidal winding machine to ensure absolute consistency throughout the windings. As Rolls Royce would say, to enquiries about how much engine power their cars possess, they are adequate sir. In other words the transformer is built to perform, not to a price. The price is worked out once the desired performance has been achieved.

Transformers you can buy off the shelf are built to a price and a number of compromises are used to give perceived high efficiency and value for money. The main problems I have had with these low cost transformers are that they are easily shocked into core saturation because of their high flux density. As Mike said there is no air gap in the core to help alleviate any core saturation effects on switch on or for that matter on transient load currents. Once the energy storage capacitor is charged up the load will continuously discharge them at a rate determined by the capacitor size and the load current. The transformer will only add charge once the voltage across the capacitor drops below the rectified input voltage minus the rectifier forward voltage drop at the rate of 50 times a second in the UK (60 in the USA). If the capacitor voltage has dropped say 1 volt lower than the incoming rectified voltage in this time, and the transformer secondary winding impedance is say 0.1 ohms, this will require a 10 amp surge from the transformer at the beginning of each charge period when the rectifiers switch on. I am not entirely happy with the effect that current surge suppressors have on transformer voltage regulation during these transient currents. Also these low cost transformers usually buzz like a hornets nest, especially in the larger core sizes, which is annoying when listening to music.

The SR5 transformer is virtually silent in operation.

Out of interest, if you are using a bridge rectifier for your rectification, you will also need to de-rate the transformer AC current delivery by a factor of 0.6 to arrive at the continuous DC current rating for the power supply. You will also need to allow for periods of heavy load on the grid, which can cause low mains voltage. I usually allow for a 15% mains voltage drop to ensure the regulators have enough headroom under these circumstances.

Regards
Paul

[Marco]

Hi Paul,

Just a quickie regarding transformers... You wrote:

The transformer I use in the SR5 is rated at 160 VA but it is wound on an oversize core with a lower flux density than typical “off the shelf” transformers. The secondary voltage is 25 volts AC. The cores and windings are carefully manufactured from high quality materials. The way the transformer is wound is also a large part of the performance and these transformers are manually wound on a toroidal winding machine to ensure absolute consistency throughout the windings.

The output transformers on my TD copper valve amp are hand-wound by a 'transformer guru', whom I believe was ex-GEC employee, and so not done by a machine.

What sonic advantages do you (or others here) perceive hand-wound transformers offer over those that are machine-wound?

Regarding the emboldened text above, I would've thought that a machine would wind transformers with more consistency than a human being, as if set correctly, they have presumably less margin for error - or not?

Educate me here!

Marco.

[Mike_New]

Hey Paul
Maybe there is a small extra market for you, selling you transformers to the cognoscenti