I have two questions. In the picture, it calls for a 12nf cap across the 6.8k resistor.
I have some 18nf caps in my collection. Would it hurt/degrade the amp if I were to
use the 18nf caps? in place of the 12nf caps?
Question 2, how do you set the bias on this amp? I cannot find any documentation
on it.
Thanks
We need the full schematic and I found it here:
https://s1.bukalapak.com/img/6633466421/large/ETI_Module_SC480_PCB_Fiber_Premium.png
The 12nF cap is in series with the collector of Q4 and I would be surprised if it makes much difference, even if it was missing. So, anything similar should do, ie 1n to 100n.
The idle current bias is obviously VR1, and there is no offset adjustment. The offset voltage should be insignificant, but it could possibly be improved by moving the input 22k to the other side of the 2.2K, or changing it to 20K so that the total bias resistance is the same on both sides of the LTP.
This is a pretty typical PP VAS circuit but the 10pF on the negative side of the LTP is questionable. However, it may be a required hack to make the real circuit work. The output drivers should be cross coupled, i.e. one resistor from the 3055 bases to the 2955 bases, with maybe a 100nF cap "speed-up" across it, instead of the two 100 Ohm resistors. This and the choice of 3055/2955 shows the age of this circuit, although maybe it is intended for those who do not have access to newer parts.
I would not use these parts (3055/2955) with a +/-40V supply. The SOA of these parts is not good enough for that high voltage. They will die. Either run it at no more than +/-25V or choose better parts. Old part that are suited to +/-40 V supply were 2N3773/2N6609. Popular parts today are 2SC5200/2SA1943 but they use a different plastic package and they are much faster so there may be stability problems. The TO-3 package is no longer popular but you can find newer suitable parts using the on-semi selector guide.
This circuit does not have any short circuit protection, another reason it will not live long. Adding a current limit is easy and only requires a couple more parts. The standard circuit(s) for current limit is shown on many other circuits on the web.
https://s1.bukalapak.com/img/6633466421/large/ETI_Module_SC480_PCB_Fiber_Premium.png
The 12nF cap is in series with the collector of Q4 and I would be surprised if it makes much difference, even if it was missing. So, anything similar should do, ie 1n to 100n.
The idle current bias is obviously VR1, and there is no offset adjustment. The offset voltage should be insignificant, but it could possibly be improved by moving the input 22k to the other side of the 2.2K, or changing it to 20K so that the total bias resistance is the same on both sides of the LTP.
This is a pretty typical PP VAS circuit but the 10pF on the negative side of the LTP is questionable. However, it may be a required hack to make the real circuit work. The output drivers should be cross coupled, i.e. one resistor from the 3055 bases to the 2955 bases, with maybe a 100nF cap "speed-up" across it, instead of the two 100 Ohm resistors. This and the choice of 3055/2955 shows the age of this circuit, although maybe it is intended for those who do not have access to newer parts.
I would not use these parts (3055/2955) with a +/-40V supply. The SOA of these parts is not good enough for that high voltage. They will die. Either run it at no more than +/-25V or choose better parts. Old part that are suited to +/-40 V supply were 2N3773/2N6609. Popular parts today are 2SC5200/2SA1943 but they use a different plastic package and they are much faster so there may be stability problems. The TO-3 package is no longer popular but you can find newer suitable parts using the on-semi selector guide.
This circuit does not have any short circuit protection, another reason it will not live long. Adding a current limit is easy and only requires a couple more parts. The standard circuit(s) for current limit is shown on many other circuits on the web.
This circuit appeared in Silicon Chip magazine issue of January 2003 I have kept this but not that of February where the set up procedure was described. The design was an update of a module published in 1987 which used 2N3055 and MJ2955 in case builders of that module wanted to use those devices in the new module.
According the article countless SC modules had been built since 1987 and the popular and even earlier ETI 422 I have that article in an audio projects compilation of early 1970's items.
A friend of mine built the ETI 422 using these transistors in single pairs. I built the 1976 Electronics Australia Playmaster twin 40 a quasi complementary design which used +/-35 Volt rails with 2N3055 outputs. Neither of us had any output transistor failures.
The SC480 doubles up on output pairs. 2N3055 and MJ2955 are general purpose transistors made by various companies. Semiconductor sales is a competitive business so one or more maker might uprate the specs and all of them would have to match the new standard.
There is a lot of attention to detail in the SC480 with among other things such as PCB layout to cancel asymmetric magnetic fields - a major cause of harmonic distortion.
Re set up procedure I looked at a later Silicon Chip design - the Studio350 from January 2004. In this the fuses are replaced by 10 Watt 470R resistors and the adjustment pot is turned anticlockwise for the lowest standing current. The dc offset is checked and the total current draw of a module can be determined from Ohm law against the 470R resistors. For SC480 5Watt 100 R resistors should be OK.
I would kept a finger on the power switch and one on the body of the resistor before taking any measurements across these resistors - they will get hot if there is a major fault. If OK the next step is to monitor the voltage drop across R1 or R2 for the desired Output stage Iq. A 5 mV drop will give a little over 20 m.a. I suggest that as a starting point - you will have to check and adjust that periodically over the time it takes for the amplifier to warm up.
The output in the SC480 has a PTC thermistor which will increase in resistance if there is a short on the amplifier output.
According the article countless SC modules had been built since 1987 and the popular and even earlier ETI 422 I have that article in an audio projects compilation of early 1970's items.
A friend of mine built the ETI 422 using these transistors in single pairs. I built the 1976 Electronics Australia Playmaster twin 40 a quasi complementary design which used +/-35 Volt rails with 2N3055 outputs. Neither of us had any output transistor failures.
The SC480 doubles up on output pairs. 2N3055 and MJ2955 are general purpose transistors made by various companies. Semiconductor sales is a competitive business so one or more maker might uprate the specs and all of them would have to match the new standard.
There is a lot of attention to detail in the SC480 with among other things such as PCB layout to cancel asymmetric magnetic fields - a major cause of harmonic distortion.
Re set up procedure I looked at a later Silicon Chip design - the Studio350 from January 2004. In this the fuses are replaced by 10 Watt 470R resistors and the adjustment pot is turned anticlockwise for the lowest standing current. The dc offset is checked and the total current draw of a module can be determined from Ohm law against the 470R resistors. For SC480 5Watt 100 R resistors should be OK.
I would kept a finger on the power switch and one on the body of the resistor before taking any measurements across these resistors - they will get hot if there is a major fault. If OK the next step is to monitor the voltage drop across R1 or R2 for the desired Output stage Iq. A 5 mV drop will give a little over 20 m.a. I suggest that as a starting point - you will have to check and adjust that periodically over the time it takes for the amplifier to warm up.
The output in the SC480 has a PTC thermistor which will increase in resistance if there is a short on the amplifier output.
We've posted alternate viewpoints here, mjona, but that won't hurt:
SC480 was designed as a cheap alternative to the current (circa 2000) trend to "blameless" type hi-fi designs. There was nothing cheap about the MJL series power transistors and their drivers then - MJL3281 and complement cost near A$20 ea. retail and a full stereo set of semis might have cost a week's wages. Reverting to the only cheap, widely sold local alternatives (2N3055/MJ2955) greatly reduced the cost to affordable for almost anyone. That's still the case in some countries. Whilst a wide range of alternative and NOS semis has always been available in the US, it's likely they would need to be airfreighted here and that makes them as expensive as the current premium audio types. It's ol' faithful or current tech here - not much in between.
The amplifier does work well as designed, is quite stable and sounds remarkably good for a utility type of amp. There were support kits for a rugged, discrete semi, output protection relay and a basic preamp with Baxandall tone controls or you could have the fancier versions developed for the earlier Blameless designs. Obviously, relays weren't included on the amplifier boards as they aren't shown on the schematic.
The core design goes back a long way to the E-I transformer era when there were plenty of cheap, 2 x 28V 2-2.5 A rated transformers everywhere. The resulting output was a nominal +/- 40VDC that sagged under load even with a single pair of output transistors but this worked fine for safe and reliable use with guitar, PA and modest hifi amps. It still does for countless numbers of people around the globe. If you visualise the supply as a rigid +/- 40V, you'll miss the key to the design's integrity.
I wouldn't disagree that you could design a better product both then and now but the over-riding concern for a magazine project is addressing popular wants at an attractive low cost. Press the wrong buttons of your readers and the business soon goes down the gurgler.
SC480 was designed as a cheap alternative to the current (circa 2000) trend to "blameless" type hi-fi designs. There was nothing cheap about the MJL series power transistors and their drivers then - MJL3281 and complement cost near A$20 ea. retail and a full stereo set of semis might have cost a week's wages. Reverting to the only cheap, widely sold local alternatives (2N3055/MJ2955) greatly reduced the cost to affordable for almost anyone. That's still the case in some countries. Whilst a wide range of alternative and NOS semis has always been available in the US, it's likely they would need to be airfreighted here and that makes them as expensive as the current premium audio types. It's ol' faithful or current tech here - not much in between.
The amplifier does work well as designed, is quite stable and sounds remarkably good for a utility type of amp. There were support kits for a rugged, discrete semi, output protection relay and a basic preamp with Baxandall tone controls or you could have the fancier versions developed for the earlier Blameless designs. Obviously, relays weren't included on the amplifier boards as they aren't shown on the schematic.
The core design goes back a long way to the E-I transformer era when there were plenty of cheap, 2 x 28V 2-2.5 A rated transformers everywhere. The resulting output was a nominal +/- 40VDC that sagged under load even with a single pair of output transistors but this worked fine for safe and reliable use with guitar, PA and modest hifi amps. It still does for countless numbers of people around the globe. If you visualise the supply as a rigid +/- 40V, you'll miss the key to the design's integrity.
I wouldn't disagree that you could design a better product both then and now but the over-riding concern for a magazine project is addressing popular wants at an attractive low cost. Press the wrong buttons of your readers and the business soon goes down the gurgler.
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Thanks to all for your answers! I replaced the 3055/2955 transistors with TIP35C & TIP36C because
I had them left over from another project of years ago. I also subbed a 2N3810 dual PNP transistor
for the input stage (had them left over too). I'm still waiting on a couple of parts to arrive before I
can fire it up. I built it to use in a bi-amp arrangement with the SC-200's that I've built as monoblocks
and they sound (at least to me) great.
I had them left over from another project of years ago. I also subbed a 2N3810 dual PNP transistor
for the input stage (had them left over too). I'm still waiting on a couple of parts to arrive before I
can fire it up. I built it to use in a bi-amp arrangement with the SC-200's that I've built as monoblocks
and they sound (at least to me) great.
The ETI 422 used a 1.5A 2x28V transformer, This was well before the era of CD, compressed music and the loudness wars. ETI rated this at 50W where distortion was 0.27% at 100Hz, at 1W it was 0.14%. at 6.3kHz this reduced slightly to 0.12%. I did not have any problem with the sound my friends one produced - my hearing was more acute back in the 1970's.
If the aim of the SC480 was to offer a simple design as an alternative to Self's Blameless amplifier using 13 readily available transistors - easy to build for modest outlay, Audio Precision test gear graphs suggest the results are good enough for most people. I think much credit is due to the board layout.
Upgrading the transformer to 2.4A allowed an output of 60W into 4 Ohms with THD below 0.01% up to 2kHz rising to 0.005% at 20kHz - these TO-3 versions with economical drivers and outputs made by MOSPEC.
This amplifier was not offered as kit in this country so I did not build one in either form. The TIP3055 and TIP2955 TO-218's are better - 0.003% at 20kHz.
I doubt the differences between versions would be audible.
The LTP dual transistors make a nice touch - should be an improvement if hFE is better than the broad specs suggest and depending on the grade and source.
I've no experience with using TIP35/36 in this design. They don't have a lot in common now, with original epitaxial 2N3055/2955 types but none of them are the same as they were when introduced so I think you'll be finding out for yourself unless another member has something on the likely versions you have from years ago. I've no doubt they'll work but its wise to be aware of differences that are going to bite you sometime.
TIP35C/36C are fun little guys.
Somewhat notorious for surviving abuse.
A favorite Gen Purpose for the MI industry.
125 watt package, but rated at 25 amps and can take 50 amp punches.
so tend to survive when the really shouldn't LOL
Somewhat notorious for surviving abuse.
A favorite Gen Purpose for the MI industry.
125 watt package, but rated at 25 amps and can take 50 amp punches.
so tend to survive when the really shouldn't LOL
Universally better than 3055’s in applications where the epi 3055 would otherwise be used. The 50 amp peak current capability ensures that you actually have GAIN above 10 amps. 3055’s often have single digit hFE up there. Blow the drivers and you blow the outputs. Not to mention the distortion.
That's more or less what the designer/editor Leo Simpson claimed. I built the TO218 version accordingly but mostly because I'm lazy and cheap anyway. There is a free download of either PCB pattern at the Silicon Chip website by searching the article in the shop, so you can see the layouts but you won't get access to the full article(s) so easily ($$). https://www.siliconchip.com.au/Shop/?article=3990
Where are you guys puchasing power transistors? I want to try using some NJW21194G/NJW21193G output transistors but can't find them anywhere.
I sued MJW not NJW in my SC200's. Just a slightly larger package.
eBay has many listings.
Thanks. They have ST TIP35/36 transistors in stock. Looks like I willl have to use those as replacements. Not keen on using ebay or aliexpress as I don't want fakes.
The trouble with the Avnet group companies like Newark, Element 14 and RS components now, is their stockholdings are stupidly small or effectively non-existent, so you have to wait until they receive the stuff from their supplier(s) first. Sure, it does arrive eventually but piecemeal and way slower than before the business mergers and our reasonable expectations for a large global organisation. I'm waiting up to months for some items in a several item order and the part delivery costs plus the hit to whatever reputation remains, must be far more significant than using smarter distribution measures. Whatever the product quality assurance, their service now sucks.
If your order is enough to meet their minimum value requirement, Digikey or Mouser will deliver free in a shorter space of time. I'm in regional Oz and I've had deliveries from Digikey via their Singapore base in as little as five days.
If your order is enough to meet their minimum value requirement, Digikey or Mouser will deliver free in a shorter space of time. I'm in regional Oz and I've had deliveries from Digikey via their Singapore base in as little as five days.