Power transformer as audio transformer
I'd like to hear someone having tried to use a power transformer as output
transformer for the single-ended final stage of a tube communications receiver (power 5W, maximum, frequency not higher than 4,500 Hz). As a matter of fact a plain power transformer with a 220V primary winding and 6V secondary winding would well match a 4-ohm speaker to a 5400-ohm plate load. Basic questions: 1) does the DC current flowing through the transformer create problems? (do proper audio tranformers have a gap in the magnetic path to avoid saturation?) 2) would a typical transformer work up to 4500 Hz, while maintaining the expected impedance ratio? 3) would modern tranformers wound on a toroidal core offer better performances? Unfortunately, proper audio tranformers are getting scarce these days. Thanks and 73. Tony I0JX |
Power transformer as audio transformer
Antonio Vernucci wrote:
I'd like to hear someone having tried to use a power transformer as output transformer for the single-ended final stage of a tube communications receiver (power 5W, maximum, frequency not higher than 4,500 Hz). As a matter of fact a plain power transformer with a 220V primary winding and 6V secondary winding would well match a 4-ohm speaker to a 5400-ohm plate load. If it were a push-pull stage, it would be easy to say yes. You can do it with some transformers, not with others. The frequency response won't be wonderful, but it will be fine for communications work. But with a single-ended stage, forget it. You need a transformer that is designed for impressed DC. You might be able to take a filament transformer and cut a gap in the core, but the result will not be predictable and it will take a lot of trial and error to get it right. Basic questions: 1) does the DC current flowing through the transformer create problems? (do proper audio tranformers have a gap in the magnetic path to avoid saturation?) Yes, massive ones. 2) would a typical transformer work up to 4500 Hz, while maintaining the expected impedance ratio? Many will. Some modern transformers won't... the Talema toroids have very poor high frequency response, deliberately to reduce power line trash from getting into your power supply. Most cheaper E-I core types will work. 3) would modern tranformers wound on a toroidal core offer better performances? Depends on what the core is made of. Unfortunately, proper audio tranformers are getting scarce these days. Hammond is still around! --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
Power transformer as audio transformer
"Antonio Vernucci" wrote in message Tony I0JX Tony, this has been discussed numerous times on the AntiqueRadios.com forums. Yes, it will work, and apparently it works quite well by all reports. The problem is core saturation, as you suspected. You will probably get a bit less audio (before clipping) than you anticipated for a given core size. If you have a few filament transformers, it might be well worth your time to experiment and try it for yourself. Since most filament transformers are designed for secondary currents in amps, it is logical that they will handle a watt of audio without much issue. Increasing the gap between E and I core laminations will to reduce saturation will dramatically reduce the low frequency response. 73 Peter K1ZJH |
Power transformer as audio transformer
Antonio Vernucci wrote:
I'd like to hear someone having tried to use a power transformer as output transformer for the single-ended final stage of a tube communications receiver (power 5W, maximum, frequency not higher than 4,500 Hz). As a matter of fact a plain power transformer with a 220V primary winding and 6V secondary winding would well match a 4-ohm speaker to a 5400-ohm plate load. Basic questions: 1) does the DC current flowing through the transformer create problems? (do proper audio transformers have a gap in the magnetic path to avoid saturation?) 2) would a typical transformer work up to 4500 Hz, while maintaining the expected impedance ratio? 3) would modern transformers wound on a toroidal core offer better performances? Unfortunately, proper audio transformers are getting scarce these days. Thanks and 73. Tony I0JX A 6.3V filament transformer will work but will be far from ideal. Far better to take one from an old valve broadcast receiver. Steve H |
Power transformer as audio transformer
On Sat, 9 Aug 2008, Antonio Vernucci wrote:
I'd like to hear someone having tried to use a power transformer as output transformer for the single-ended final stage of a tube communications receiver (power 5W, maximum, frequency not higher than 4,500 Hz). I haven't done it, but it was a pretty common trick to see in the magazines in the old days. At least one article pointed out that if you started with a 400Hz transformer, which were common in surplus from aircraft, you'd likely get better frequency response where you needed it. Michael VE2BVW |
Power transformer as audio transformer
Thanks everyone for the replies.
However no one touched the issue whether the proper output transformers have E and I laminations arranged so as to create a gap (i.e. like non-swinging filter chokes). I would not like having to disassemble an audio transformer just for the sake of determining whether there is a gap in the magnetic path or not. Buying audio transformers is not cheap today: - buying in the US implies a minimum shipping charge of about 30$ (international USPS rates have gone crazy. Forget UPS or Federal Express) - buying on e-bay not easy either! You'll find yourself within hordes of esoteric hi-fi aficionados prepared to pay almost any price for plain things. If you wish to buy a Silver Rock audio transformer ( http://www.mantrasound.it/AC_components/AC2.htm ) be prepared to pay at least 4,200 Euro (about $6,300 with the US$ now getting stronger). 73 Tony I0JX |
Power transformer as audio transformer
http://www.antiqueradios.com/forums/...a99607a05f2b09
http://www.antiqueradios.com/forums/...61700 d5d187f http://www.antiqueradios.com/forums/...a99607a05f2b09 Some reading regarding this subject. Pete |
Power transformer as audio transformer
"Tio Pedro" ha scritto nel messaggio
... http://www.antiqueradios.com/forums/...a99607a05f2b09 http://www.antiqueradios.com/forums/...61700 d5d187f http://www.antiqueradios.com/forums/...a99607a05f2b09 Some reading regarding this subject. Pete Thanks, reading those discussions and other references, it was possible to determine that single-ended audio transformers must have a gap in the magnetic path, this fact making power transformers generally unsuitable for that application. 73 Tony I0JX |
Power transformer as audio transformer
The same problem with the DC flow in the transformer exists when a power transformer is used with a halfwave rectifier. Typically the transformer rating is degraded by 60% when be using with halfwave rectication due to the DC current flowing in the secondary. Many folks have used filament transformers as audio outputs replacements with good results. The transformer rating is decreased by the standing DC current, but it will still handle a good amount of audio before saturation on peaks occurs. Peter |
Power transformer as audio transformer
You can avoid direct current from flowing in the filament transformer used
as an output transformer by using a choke with sufficient inductance to act as a load, then capacitive coupling the filament transformer to transform impedances only. That way, the filament transformer is only used with ac. This method is often used with modulation transformers, to prevent dc from saturating the core of the modulation transformer. 73, Colin K7FM |
Power transformer as audio transformer
Antonio Vernucci wrote:
Thanks, reading those discussions and other references, it was possible to determine that single-ended audio transformers must have a gap in the magnetic path, this fact making power transformers generally unsuitable for that application. Yes, but it IS possible to to cut a gap into an existing EI core transformer. The problem is that when you do it, ALL of the transformer parameters will change. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
Power transformer as audio transformer
Tio Pedro wrote:
The same problem with the DC flow in the transformer exists when a power transformer is used with a halfwave rectifier. Typically the transformer rating is degraded by 60% when be using with halfwave rectication due to the DC current flowing in the secondary. No, the DC currents sum to zero. That derating is only required when the transformer secondary is poorly balanced. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
Power transformer as audio transformer
"Scott Dorsey" wrote in message ... Tio Pedro wrote: The same problem with the DC flow in the transformer exists when a power transformer is used with a halfwave rectifier. Typically the transformer rating is degraded by 60% when be using with halfwave rectication due to the DC current flowing in the secondary. No, the DC currents sum to zero. That derating is only required when the transformer secondary is poorly balanced. --scott How does that occur in a half-wave circuit? It is a series circuit. The half-wave rectifier produces a voltage across the load that is a complex waveform with both a DC and an AC component. Since it is a series circuit, the same situation exists in the transformer secondary. DC sums would be zero with a full wave rectifier or a full-wave voltage doubler. Pete |
Power transformer as audio transformer
On Aug 9, 3:04*pm, "Antonio Vernucci" wrote:
I'd like to hear someone having tried to use a power transformer as output transformer for the single-ended final stage of a tube communications receiver (power 5W, maximum, frequency not higher than 4,500 Hz). As a matter of fact a plain power transformer with a 220V primary winding and 6V secondary winding would well match a 4-ohm speaker to a 5400-ohm plate load. Basic questions: 1) does the DC current flowing through the transformer create problems? (do proper audio tranformers have a gap in the magnetic path to avoid saturation?) 2) would a typical transformer work up to 4500 Hz, while maintaining the expected impedance ratio? 3) would modern tranformers wound on a toroidal core offer better performances? Unfortunately, proper audio transformers are getting scarce these days. Thanks and 73. Tony I0JX Was looking at a smallish ex-microwave oven transformer other day and wondering same. using one at much lower current as single ended OP transformer? Typical small scrapped m.wave transformer. Thousand watts at 115 volts means primary handles around 8 to 9 amps. Secondary at say 2000 volts handles 0.5 amps (500 ma.) Suppose we were to use it at say less than 100 ma. plate current; to avoid saturation? Note: If the output of the transformer is around 2000 volts RMS (before DC voltage doubling etc.) and has a 115 volt primary it has a voltage ratio of around 17 to one. So a (single ended) tube impedance match from an 8 ohm speaker of about 2400 is possible? That could be suitable for a triode connected (non P-P) output tube? Also wondered if other power transformers could be used at low DC currents to avoid saturation etc. and perhaps cathode connected? For example a centre tapped 700 volt power transformer (350-0-350) would have a voltage ratio to its 6.3 volt winding of 111, hmmm! That's a very high impedance ratio! And it's 6.3 plus 5.0 volt winding (11.3) it would have a voltage ratio of about 62, also very high! But to its 115 volt winding the voltage ratio is 700/115 = 6.1 for an impedance transform of 37. Thence 37 times 8 = 296; adequate to match the cathode follower impedance of P-P 'grounded' plate OP tubes? With DC currents in the cathode circuits tending to cancel each other? Just playing with thoughts at this stage! Note: Seen use of those ex. m.wave transformers for battery chargers, their primaries are pretty rugged and a 12 volt secondary can be not too many turns of something capable of quite a few amps! |
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