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In writing to Hafler authorized repair technicians about my aging 915 preamp and 9300 power amp, I have recieved the following comment:"One thing that you need to know is that the 915 has a solid state input selector switch that is operated by the front panel control. That is a sonic "bottleneck" but the tape input does not pass through that switch. The tape input is relay controlled and consequently will provide cleaner sound than the other inputs on the 915."
The Hafler manual states that this CMOS-FET electronic switch is a sonic advantage because it eliminates signal degradation due to mechanical switch contacts and allows the electronic switches to be located near the rear panel inputs for a shorter signal path and reduced interchannel crosstalk.
The tape input, of course, has a mechanical switch as stated by the technician.
What are the relative sonic advantages and disadvantages of the CMOS-FET switch compared to the mechanical switch? Do you agree with the technician that a mechanical switch must deliver cleaner sound?
Is the CMOS-FET switch a cause for concern?
Follow Ups:
I did a listenging test to compare the tape monitor relay input to the selector switch CMOS-FET input. Graham Slee, Janis Ian, Paul Simon and Barbirolli conducting Sibelius 2nd provided the short program.If there is a difference between these two inputs, it is not much. Certainly a tiny amount compared to my recent switch from MAS Gray interconnect to MAS Blue. If there is a difference, the selector switch CMOS-FET connection is better. Maybe a little more low level detail and a little better rendering of space. Maybe not.
With the system as it is now, the MAS Blue is much more pleasing on the Slee phono preamp. The Gray is too forward in the upper midrange and lower treble. I guess I attribute some of this to the Hafler 9300 power amp. I did the testing with the Gray interconnect, which does etch the details and control the bass better than the Blue.
If this selector switch is "cheap and cheerful" as the Brits say, then cheers for Hafler using it to increase profit without hurting the sound. They STILL had to drop out of the hi-fi hype market shortly after making this preamp and are now kaput.
I also asked John Hillig about the selector switch, since I still plan to have this preamp modified by him. He wrote back as follows.
"Modified 915s sound pretty good. I too was worried about the FET switching but I have a better view of it now. "
HI Dave,
I have had this argument with myself and mechanical switch contacts won. I include relays with mechanical switch contacts. If your system is capable of very high resolution and you enjoy the "little sounds" that analog offers, you will find the same thing, I think.If you research CMOS switches in data books they seem ideal for audio switching. But, listening tests tell me that the spec sheets don't tell the whole story. I hear degradation. CMOS switches soften and smooth over the presentation. That's what I have found.
Mechanical switches are not perfect either. They can and do get dirty and/or oxydized which can have serious sonic effects. Most of the time they can be restored to like new using Caig Laboratories Deoxit. Great stuff. These folks should get a Nobel prize for Audio.
One of the things I look for in new equipment is relay switching. They combine the reliability of CMOS switches and the sonics of mechanical switches. Please note that my new Krell preamp, used in my HT system, and Aesthetix phono preamp uses relay switching. Since CMOS switches are less expensive to implement and require far less space, I think this tells the whole story. For relays to live up to their promise, they must be hermetically sealed. If not sealed the relay contacts will suffer from dirt and oxydation just like mechanical switches. And they are a bitch to clean. Fortunately, almost all relays used in audio equipment are sealed.
Sparky
Thanks for your answer, Sparky.The manual further describes the Input Selector switch as having its output followed by a very high input-impedance JFET buffer pair to establish negligible CMOS switch loading for low distortion and to provide a low impedance source for the Record Output and Volume and Balance controls. Does this change your opinion of the CMOS switch?
The tape moniter switch is described as activating a relay with gold plated contacts.
I have yet to do a listening test, but have already tried, in vain, to find out if there is an audiophile grade line level switchbox being made with two or three inputs. Two is all I really need for LP and SACD. I guess that this is something that a DIYer could make with a high quality switch (or a relay as you describe would be better), solder and wire, but I am not endowed with those skills.
If anyone reading this string knows of such a switchbox, or could be hired to make me one, I would appreciate an email with the info.
I really like the intelligently designed tone controls on this preamp, since I listen to many recordings that are not perfect. The ability to have some control over bass and treble makes many of these recordings enjoyable.
Maybe CMOS switches suffer from unclean power supplies.
AFAIK the 4066 switches with one CMOS transistor, how many more are there in an opamp?
Wouldn't cleaning up the crap coming into the gate of the CMOS improve the sound overall?
just my 2 c
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