I wouldnt assume that 10k would be the best value. But probably a good safe starting place. Have a look at the datasheet if there is one.
The cinemag data sheet shows that the input impedance varies with load.
I suspect the 15k rating is a nominal rating, a bit like a loudspeaker, but as its a coil its sensitive to ac signals.
I the values on the datasheet examples range from 10k to 15k and I have an 18k resistor with mine.
Theres also space for a capacitor in series with the resistor which isnt used, just shorted.
There were some pictures somewhere, possibly on this thread somewhere. You put a square wave in and using a scope, experiment with different values of resistors and capacitors to see which give the the best loading. I think this means getting the output as square as possible without too much overshoot at the top of the rising edge.
Maybe solder some pins into to your circuit to make quick temporary connections to your board easy and less dangerous to the board.
http://www.sowter.co.uk/pdf/3575.pdf
Looking here
DIY Test Equipment for Audio and Ham Radio Enthusiasts
It looks like the 18k metal film is still used on the input.
Hope this helps.
The cinemag data sheet shows that the input impedance varies with load.
I suspect the 15k rating is a nominal rating, a bit like a loudspeaker, but as its a coil its sensitive to ac signals.
I the values on the datasheet examples range from 10k to 15k and I have an 18k resistor with mine.
Theres also space for a capacitor in series with the resistor which isnt used, just shorted.
There were some pictures somewhere, possibly on this thread somewhere. You put a square wave in and using a scope, experiment with different values of resistors and capacitors to see which give the the best loading. I think this means getting the output as square as possible without too much overshoot at the top of the rising edge.
Maybe solder some pins into to your circuit to make quick temporary connections to your board easy and less dangerous to the board.
http://www.sowter.co.uk/pdf/3575.pdf
Looking here
DIY Test Equipment for Audio and Ham Radio Enthusiasts
It looks like the 18k metal film is still used on the input.
Hope this helps.
A transformer will just transform the load, so yes, the input impedance will change as you change that resistor. However, the transformer reactances will cause ringing or rolloff if that load significantly deviates from the recommended one. For a transformer optimized for 10k, using a 100k load will result in some overshoot, thus the Zobel to control this. In the article, I gave the Zobel values for the Jensen, but they'll likely be different for a Sowter.
I've been using the preamp for a few days and I'm convinced the sounds improving. There seems to be a lot more space in the treble and an improved sound stage. Last time I was talking about burn in was my f4 and I was told it only needed a few seconds. Is it the caps in the audio path with the impasse, as both my cd player and my sonos box all exhibited this improvement over the first 20 or 30 hours or so.
I'm astonished at how much difference a better amp has made. Its replaced an Arram delta 90.2 that had a pretty solid sound. I thought that the most important thing was source and didn't expect the sound stage improvement that has been delivered.
I'm astonished at how much difference a better amp has made. Its replaced an Arram delta 90.2 that had a pretty solid sound. I thought that the most important thing was source and didn't expect the sound stage improvement that has been delivered.
A few questions regarding a power transformer, I'm a total tube newbie so please bear with me.
Is this a good candidate for a power transformer for the Impasse? https://www.hammfg.com/files/parts/pdf/369BX.pdf
Even though it is a CT secondary, is it ok to only use the 2*150V taps and get 300VAC secondary? 300VAC is about 420VDC after rectification so it does meet the criteria, or am I missing something?
Can I use one heater secondary to heat both tubes?
Is a 2.3mm (almost 0.1 inch) gap between the top of the transformer and the enclosure cover plate (anodized aluminum) enough for isolation? If not, the option is to mount it on a 90º angle bracket laying down. Any considerations if mounting sideways?
Is this a good candidate for a power transformer for the Impasse? https://www.hammfg.com/files/parts/pdf/369BX.pdf
Even though it is a CT secondary, is it ok to only use the 2*150V taps and get 300VAC secondary? 300VAC is about 420VDC after rectification so it does meet the criteria, or am I missing something?
Can I use one heater secondary to heat both tubes?
Is a 2.3mm (almost 0.1 inch) gap between the top of the transformer and the enclosure cover plate (anodized aluminum) enough for isolation? If not, the option is to mount it on a 90º angle bracket laying down. Any considerations if mounting sideways?
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I don't mean to get ahead of myself, but see there are Impasse parts kits from the techdiy which were not in stock the last time I checked but appear to be back in today (please stay in stock until mid-February!). To prevent the future suffering and the clutter it would cause as I would need to post a few dozen help questions - I'm interested in purchasing the completed and tested boards with Cinemag transformers and the assembled power supply. Yes, it is "less" DIY, but I got over my head with this project some 15 months ago. It is worth the cost and partial reassurance.
Is this a good source or should I keep looking?
What other obstacles and expenses am I likely to run into with this build? It's for a pair of F4 mono blocks under construction using boards purchased from the DIY Audio store.
One guess that might negate the "pre-tested" aspect:
The voltage at my home runs high for the US, usually at 125V. After a neighborhood transformer blew it dropped to 121V but the power company "fixed it" and I'm back at a full 125.0V. It's not something I have control over.
I don't have the tools for measuring wave's, etc. A few DMM's and not much more.
Today I'm putting together a source selector switch and potentiometer in a box to declutter a preamp build, might even add a second selector switch on the output for a choice of pre-amp / amp combinations. I haven't asked what "Eliminate Alps and selector switch" means.
Is this a good source or should I keep looking?
What other obstacles and expenses am I likely to run into with this build? It's for a pair of F4 mono blocks under construction using boards purchased from the DIY Audio store.
One guess that might negate the "pre-tested" aspect:
The voltage at my home runs high for the US, usually at 125V. After a neighborhood transformer blew it dropped to 121V but the power company "fixed it" and I'm back at a full 125.0V. It's not something I have control over.
I don't have the tools for measuring wave's, etc. A few DMM's and not much more.
Today I'm putting together a source selector switch and potentiometer in a box to declutter a preamp build, might even add a second selector switch on the output for a choice of pre-amp / amp combinations. I haven't asked what "Eliminate Alps and selector switch" means.
Ah, forgot that was your store and that you are in the US.
Just trying to get a better idea of what parts will be needed and clarification on "eliminate alps pot and selector" with what appears to be a fee to leave parts out as it appears on the page to someone that probably has some misunderstanding.
Just trying to get a better idea of what parts will be needed and clarification on "eliminate alps pot and selector" with what appears to be a fee to leave parts out as it appears on the page to someone that probably has some misunderstanding.
Jumper C1? -- maybe not.
Running a square wave it seemed that 15kΩ did a good job taming the ringing. Nevertheless, I observed a disappointing attenuation of the signal.
I ran the spectrum from 5kHz to 100kHz and observed the peak
Using C=1/2πRf = 390pF I soldered in a 390pF -- seems to be a good tradeoff.
Running a square wave it seemed that 15kΩ did a good job taming the ringing. Nevertheless, I observed a disappointing attenuation of the signal.
I ran the spectrum from 5kHz to 100kHz and observed the peak
Using C=1/2πRf = 390pF I soldered in a 390pF -- seems to be a good tradeoff.
Attachments
I'm currently running my impasse into my f4 single ended. I was wondering why you have to take the cold output and not the positive. Would a dummy load on the cold work in order to take the positive output? It's not a complaint, just curiosity.
I'm also considering upgrading the output DC blocking cap. It's unlikely that I would build a second amp for a long time as it's driving my obl15 speakers just fine so is there any reason I shouldnt only upgrade the cold output caps and leave the stock wima ones on the hot output which is shorted to ground via the xlr cable.
Cheers
I'm also considering upgrading the output DC blocking cap. It's unlikely that I would build a second amp for a long time as it's driving my obl15 speakers just fine so is there any reason I shouldnt only upgrade the cold output caps and leave the stock wima ones on the hot output which is shorted to ground via the xlr cable.
Cheers
When you load a cathodyne unequally, the "magic" of the source impedance being low and equal at cathode and plate disappears. So if you're only driving out of one node, you use the cathode so that the source impedance is low. Take it off the plate and you'll see significant high frequency rolloff.
Thanks sy. So what would happen if you put an appropriate resistor across the cathode and tried to drive off the anode. Obviously from what you've said it's going to be better driving from the cathode and bypasses lots of potential issues eg only working with the power amp you've matched the resistor to, but does that mean the input impedance of an amp varies with frequency? I suppose I assumed that the nominal impedance of an amplifier was high and you tied to ground via say a 10k resistor.
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Well, it's a delicate balance- the load is the interconnects and the amp being driven. Trying to get that closely matched... no, not something I'd recommend. The cathode output will work perfectly, just ground the plate's output (on the far end of the coupling cap!). Why do things the hard way and compromise performance?
OK! Point taken. I figured there was loading requirements when I read the article. In which case if I ever do decide to build a second amp I should probably match my input resistors.
Am I OK to replace just the cathode caps with some paper in oils. Do you think it'll make a lot of difference to the wimas, any recommendations?
Am I OK to replace just the cathode caps with some paper in oils. Do you think it'll make a lot of difference to the wimas, any recommendations?
At best, no difference. At worst, higher distortion- paper is a notoriously poor dielectric because of moisture uptake, and its low tensile strength means windings aren't as tight. If you want the preamp to be neutral and not an effects box, use industrial grade polyprop caps like Wima, Vishay, Panasonic, or the like.
Jack, I gotyour new pcb and try to find therightbom for it incl. the right wattage per resistor etc....can you provide some more details, please ? thx
Those newer Maida boards are designed so that you can place TWO capacitors in series. If you do this, you should place leveling resistors in parallel with each of the capacitors. I did this because the voltage coming off the rectifiers may be much higher than 450VDC, (depending upon your transformer.)
The leveling resistors can be 220K, 270K etc. Two in series will draw a few milliamps so will burn around 1W -- thus a 2W or3W resistor will be satisfactory.
The leveling resistors can be 220K, 270K etc. Two in series will draw a few milliamps so will burn around 1W -- thus a 2W or3W resistor will be satisfactory.