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901 equaliser voltage

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Guest samiam

I have the option to purchase some 901 series 2's with an American 110volt equaliser. Here in new Zealand we are 240v. Hence they are really cheap. Rather than getting a step-up transformer..... What is the internal voltage of the EQ so I can get the correct transformer to do the job.

While I'm poking around inside the EQ is there anything else I can/should do to 'tweak' it?

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I have the option to purchase some 901 series 2's with an American 110volt equaliser. Here in new Zealand we are 240v. Hence they are really cheap. Rather than getting a step-up transformer..... What is the internal voltage of the EQ so I can get the correct transformer to do the job.

While I'm poking around inside the EQ is there anything else I can/should do to 'tweak' it?

The equalizer draws very little current. The smallest step DOWN transformer designed for powering American appliances in 240V countries should be adequate. 100 VA should be more than adequate. (from your 240V NZ voltage to 120V American voltage I presume is what you need). That is your best bet. Short of contacting the Manufacturer to find out if there are additional taps on the power supply transformer or taking it apart to investigate it (I assume you would have already done that if you were technically skilled) that's the safest thing to do. Even if you found a low voltage power supply, it would need a redundant transformer large enough for that unit anyway so it would cost more.

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I have the option to purchase some 901 series 2's with an American 110volt equaliser. Here in new Zealand we are 240v. Hence they are really cheap. Rather than getting a step-up transformer..... What is the internal voltage of the EQ so I can get the correct transformer to do the job.

While I'm poking around inside the EQ is there anything else I can/should do to 'tweak' it?

Hello,

The EQ can work on both voltages by the means of a resistor in series with the transformer primary winding. The 120VAC U.S. version has no resistor, the 240VAC has a 10 Kohms / 5W wounded resistor installed in series with the mains live wire. Personnally I preferred to get rid of the original transformer and to install a native 240VAC model (the shiny copper thing on the left side). At the same time correct voltages were reinstated within the EQ, which the original transformer did not allow.

P1030166s.jpg

Full story of the rebuild and tweaking here : The Bose 901 series II equalizer tweaking

Hope it helps and it's not too late

Mr T

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Hello,

The EQ can work on both voltages by the means of a resistor in series with the transformer primary winding. The 120VAC U.S. version has no resistor, the 240VAC has a 10 Kohms / 5W wounded resistor installed in series with the mains live wire. Personnally I preferred to get rid of the original transformer and to install a native 240VAC model (the shiny copper thing on the left side). At the same time correct voltages were reinstated within the EQ, which the original transformer did not allow.

P1030166s.jpg

Full story of the rebuild and tweaking here : The Bose 901 series II equalizer tweaking

Hope it helps and it's not too late

Mr T

For all of Dr. Bose's accumen in audio science, the use of a series resistor to adjust a transformer to a different input voltage is quite frankly bad electrical engineering. In 40 years as an electrical engineer, I have never heard of anyone trying such a scheme. It is hardly surprising that the result would give the wrong output voltage. Fortunately, the output voltage was too low which may have compromised performance by biasing the transistors incorrectly. Had it been too high, it could have fried them instead. I've never examined the circuit but if the power supply output voltage is regulated, it might not matter. An external tranformer to convert 240 volts to 120 such as I described above would have been the correct choice. It is just as effective and far easier and cheaper than replacing the power supply transformer. One thing to be aware of is that there is no ventillation of the equalizer. It is important that the heat generated by the new transformer does not overheat the other internal components or they may fail prematurely. I'd remove the cover after several hours of operation and if I found it was getting too hot, I might consider drilling some ventillation holes.

Before any claims for improvement had been made, the performance of the equalizer should have been documented after the step of operating it at the correct voltage had been taken. While not necessarily a bad thing to do, replacing the other components may have been futile, any real improvement being attributable to operating the transistors at the correct quiescent operating points.

Bose 901s shortcomings were not IMO due to limitations of the equalizer but to acoustic limitations in the design of the loudspeakers themselves and the failure by many users to install them optimally. I've listed my opinions about them on many other postings. One additional limitation is the inability to adjust the direct/reflected radiation ratio, that is the balance between the loudness of the front driver versus the rear drivers. Another is the inability to independently adjust the outside reflecting panel group versus the inside. The only way to adjust these variable to compensate for the reflectivity of the walls behind the speaker was to move the speaker closer or further away from it and to angle it. Also, the FR of the rear drivers could not be adjusted independently of the front driver to compensate for variables of the frequency dependent sound absorption/reflection coefficients of reflective walls in different room. No attempt in the design was made t address any of these installation variables.

It is unfortunate that the design was not explored to its fullest potential by Bose. The decision was made at one point, the point at which series III was introduced that it would no longer be targeted at the high end audio market. The FR limitations of Bose 901 were characteristic of many other loudspeakers in the 1960s and 1970s such as its high end rolloff. Bose himself said in his white paper that other speakers he'd tried before he invented 901 sounded too shrill to him. This limitation is no longer acceptable to many serious listeners of recorded music. There are reasonable compromises between a poor high end response and the shrill piercing sound many of todays loudspeakers produce. This was a lost opportunity because the design concept of Bose 901 inherently can produce much better and more realistic high frequency reproduction than direct firing systems.

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For all of Dr. Bose's accumen in audio science, the use of a series resistor to adjust a transformer to a different input voltage is quite frankly bad electrical engineering. In 40 years as an electrical engineer, I have never heard of anyone trying such a scheme. It is hardly surprising that the result would give the wrong output voltage. Fortunately, the output voltage was too low which may have compromised performance by biasing the transistors incorrectly. Had it been too high, it could have fried them instead. I've never examined the circuit but if the power supply output voltage is regulated, it might not matter. An external tranformer to convert 240 volts to 120 such as I described above would have been the correct choice. It is just as effective and far easier and cheaper than replacing the power supply transformer. One thing to be aware of is that there is no ventillation of the equalizer. It is important that the heat generated by the new transformer does not overheat the other internal components or they may fail prematurely. I'd remove the cover after several hours of operation and if I found it was getting too hot, I might consider drilling some ventillation holes.

Before any claims for improvement had been made, the performance of the equalizer should have been documented after the step of operating it at the correct voltage had been taken. While not necessarily a bad thing to do, replacing the other components may have been futile, any real improvement being attributable to operating the transistors at the correct quiescent operating points.

Bose 901s shortcomings were not IMO due to limitations of the equalizer but to acoustic limitations in the design of the loudspeakers themselves and the failure by many users to install them optimally. I've listed my opinions about them on many other postings. One additional limitation is the inability to adjust the direct/reflected radiation ratio, that is the balance between the loudness of the front driver versus the rear drivers. Another is the inability to independently adjust the outside reflecting panel group versus the inside. The only way to adjust these variable to compensate for the reflectivity of the walls behind the speaker was to move the speaker closer or further away from it and to angle it. Also, the FR of the rear drivers could not be adjusted independently of the front driver to compensate for variables of the frequency dependent sound absorption/reflection coefficients of reflective walls in different room. No attempt in the design was made t address any of these installation variables.

It is unfortunate that the design was not explored to its fullest potential by Bose. The decision was made at one point, the point at which series III was introduced that it would no longer be targeted at the high end audio market. The FR limitations of Bose 901 were characteristic of many other loudspeakers in the 1960s and 1970s such as its high end rolloff. Bose himself said in his white paper that other speakers he'd tried before he invented 901 sounded too shrill to him. This limitation is no longer acceptable to many serious listeners of recorded music. There are reasonable compromises between a poor high end response and the shrill piercing sound many of todays loudspeakers produce. This was a lost opportunity because the design concept of Bose 901 inherently can produce much better and more realistic high frequency reproduction than direct firing systems.

Hi Soundminded,

Thanks for the comments.

Totally agree on the resistor thing. Actually the EQ runs cooler than before because :

1 - the 10K resistor was a considerable source of heat in itself : try to put your finger on it while the EQ is running and you'll get badly burned.

2 - the transformer was choosen with enough wattage capability to avoid heating (a transformer heats up accordingly with the power demand at it's secondary windings, hence many underpowered amplifiers bursting in flames when abused for a long time). Anyway that was easy because the small bunch of transistors in there need very little power.

So there is no need to worry about temperature and as it is not the first electronic device I'm working on I let it run several days on the bench with hood in place to check temperature. Meanwhile I bought tenths of BC239C transistors and selected the ones with highest HFe (gain) and I have replaced the original ones, then I can say the EQ is working as it was meant by the engineer who designed it. This is the difference between an ordinary "fall of the production line" standard device - you can say the same for cars or other manufactured equipemnt - and finely tuned products. Generally speaking this makes a big difference in sale price also.

This leads me to the following comment about necessity - or not - to upgrade this EQ, or any other audio device. Here's my point of view : a lost information cannot, ever, be recreated by the next link of the chain. In other words, starting from the audio source (CD or vynil) each device HAS to transmit as much information as possible. In best case, all of the information present in the signal, without altering it in the tiniest way. The last device before our ears is of course the loudspeaker itself and at this stage, a lost information is impossible to re-created.

All the audio equipment I have been working on (that is to says many many amplifiers, preamps, DACs) has been designed like this and it proved to work. As I have measured the EQ right after I got it second hand - i.e. in original condition - I am able to dig out the results I found and put them online so you can compare. I always measure gear before and after mods, only and honest way to conclude about relevance of the mods.

I would like to insist on the fact I really enjoy reading what you guys are writing in here, first time I read constructive discussions and comments about what is only another sound machinery, even if it's truly an unusual one. Thank you for this, and sorry again for the rough english.

Mr T

A

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Hi Soundminded,

Thanks for the comments.

Totally agree on the resistor thing. Actually the EQ runs cooler than before because :

1 - the 10K resistor was a considerable source of heat in itself : try to put your finger on it while the EQ is running and you'll get badly burned.

2 - the transformer was choosen with enough wattage capability to avoid heating (a transformer heats up accordingly with the power demand at it's secondary windings, hence many underpowered amplifiers bursting in flames when abused for a long time). Anyway that was easy because the small bunch of transistors in there need very little power.

So there is no need to worry about temperature and as it is not the first electronic device I'm working on I let it run several days on the bench with hood in place to check temperature. Meanwhile I bought tenths of BC239C transistors and selected the ones with highest HFe (gain) and I have replaced the original ones, then I can say the EQ is working as it was meant by the engineer who designed it. This is the difference between an ordinary "fall of the production line" standard device - you can say the same for cars or other manufactured equipemnt - and finely tuned products. Generally speaking this makes a big difference in sale price also.

This leads me to the following comment about necessity - or not - to upgrade this EQ, or any other audio device. Here's my point of view : a lost information cannot, ever, be recreated by the next link of the chain. In other words, starting from the audio source (CD or vynil) each device HAS to transmit as much information as possible. In best case, all of the information present in the signal, without altering it in the tiniest way. The last device before our ears is of course the loudspeaker itself and at this stage, a lost information is impossible to re-created.

All the audio equipment I have been working on (that is to says many many amplifiers, preamps, DACs) has been designed like this and it proved to work. As I have measured the EQ right after I got it second hand - i.e. in original condition - I am able to dig out the results I found and put them online so you can compare. I always measure gear before and after mods, only and honest way to conclude about relevance of the mods.

I would like to insist on the fact I really enjoy reading what you guys are writing in here, first time I read constructive discussions and comments about what is only another sound machinery, even if it's truly an unusual one. Thank you for this, and sorry again for the rough english.

Mr T

A

Negative feedback reduces differences in performance due to variations in transistor or tube characteristic curves and specifications such as h parameters from unit to unit. The gain of a negative feedback circuit is the forward gain divided by one plus the loop gain and is usually pretty constant from unit to unit of the same type. Gain bandwidth product and noise are also critical parameters. Selecting transistors based on one parameter may ignore others. Hopefully quality control at the production facility culled out of spec transistors. Mil spec transistors may have tighter tolerences, usually they are specified to operate over a wider range of temperatures. They may also be culled production batches that tested better than commercial grade transistors.

The critical filter components, the capacitors and resistors in the equalizer were all 1% and 5% tolerence. There is a threshold below which measured improvements or conversely degradations in performance are not audible. This is how audiophiles are tricked into buying expensive equipment which performs better on paper but gives no practical improvement they can use. The telltale sign of this kind of advertising is when supposed improvements are spoken about qualitatively instead of quantitatively and when double blind testing is derided as meaningless. These notions lead to absurd conclusions. For example, the typical vinyl phonograph record has had its signal processed through dozens of transistors or tubes before it reached the vinyl. Even without the recording engineer's "manipulation" the signal has been equalized at least 6 times by the time it reaches your phono output stage (2 NAB in the master and 2 in the mixdown tape decks and RIAA in the vinyl recording itself.) If it was processed with Dolby A, you can add another 8. Yet audiophile say that even one equalizer in their playback systems destroys the quality of the signal and use "passive preamplifiers" to reduce the number of active devices in their playback systems.

BTW, a transformer will generate heat if the primary windings are energized even when the secondary is not loaded. The larger the transformer, all other things being equal, the greater the heat produced. Most small signal devices like the Bose 901 equalizer do well with small transformers because they draw very little current. The test is to determine the voltage drop across the transformer secondary when it is connected to the operating load compared to when it isn't (open circuit voltage.) If it's less than about 5% or 10%, it's probably just fine for that application.

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