Sunday, December 26, 2010

The best CD4066 switch for a Quad 44 is.........

Instead of looking for the right 4066, I build a relay solution. The two relays (4 contacts) are placed on a small Pcb, piggybacked on the copper side of the Psu board.

The only awkward thing to do, is soldering a 14 pin Ic socket to this side of the board, but Quad made large soldering "islands", so with a steady hand and a small soldering tip it went ok. The relays are driven by transistors in so called “open collector mode”. The Pcb’s are degraded to prototypes because of a small non critical (repairable) layout error. How can you make an error in such a simple Pcb? Don’t ask!

In a 44 there are two switches in series with each signal path, lucky for me all the CD4066 are configured the same, so the layout of the Pcb is standard for all positions. The relays are high quality ones with gold plated contacts.

I measured THD (non weighted) and IM (Din) distortion with the Virtins analyzer (left and right channel). This is a 44 MK I with single OPA’s. All the OPA’s are BB OPA 627.

With all the original CD4066 in place:

THD 0.0055 and 0.0062 IM 0.013 and 0.015

The selector switch replaced by a relay:

THD 0.0032 and 0.0039 IM 0.009 and 0.010

The output switch replaced by a relay:

THD 0.0008 and 0.0007 IM 0.0017 and 0.0012

So a reduction of 17 dB or so of THD and IM, the relays operate click free in the signal path, but the mechanical clicks reminds me of the good old past (I am a sentimental old man!).

Altough the results of the relays are very good, the original performance of a 44 with new Op Amp's is still outstanding, considering the age of the design!

Joost Plugge

Update 8 march:

New PCB's have arived, also a connector sytem instead of a DIL socket, watch this space for the anoucement and pricing.

DaDa Electronics

Saturday, December 18, 2010

What are the best capacitors for Audio?

The quality of the capacitors, specially when they are in the signal-line, have a very big impact on the sound-quality of an audio-circuit.

There are several factors that determine the quality of a capacitor, some of them are very important for audio-applications:
  • The tolerance and the real capacitance (this is important for usage in filters)
  • The dependence of the capacitance on the frequency (1µF at 1000 Hz does not mean 1µF at 20 KHz!)
  • The internal resistance (ESR)
  • The leakage-current
  • The ageing (how will those factors evoluate over time)

The best capacitor-choice depends on the application within the circuit and the capacitance we need.

1pF to 1nF range: HF management and feedback circuits

This range is mainly used for HF-elimination in audio-circuits or for feedback purposes like in the bridge of a Quad 606 amplifier.

The best choice in this range are the Silvered Mica capacitors. They have a very good tolerance (up to 1%) and very low distortion- and noise-figures but they are rather expensive.

MKS or MKP are a good alternative. Ceramic capacitors should be avoided in the signal-line as they can cause an extra non-linear distortion of up to 1%.

1nF to 1µF: Coupling, decoupling and supression of oscillation

These are most used in audio-circuits, ao. between stages when there is a DC-level difference, to eliminate oscillation or HF-parasites and in feedback-circuits.

Typically film-capacitors will be used in this range, and even up to 4,7µF. The best choice is Polystyrol/Styrene (MKS) like Wima or Polypropylene (MKP) like Vishay. Polyethylene (MKT) is an alternative at a lower cost (but we are talking cents here).

1µF and above: Power supplies, output capacitors, filters, decoupling

In this range Electrolytic capacitors will be used. The advantage is the very high capacitance (up to 1 Farad). But there are several disadvantages:

  • Electrolytic capacitors are subject to ageing and drying out. After 10 years or more the oil is drying and the important factors like the ESR are changeing. They should be replaced every 10 years otherwise they will have a negative impact on the sound.
  • They are polarised. There should be at least 1 volt DC over an electrolytic capacitor otherwise non-linear distortion will increase to 1% or higher.

When designing a coupling-circuit electrolytes in the signal-line can often be avoided by recalculating the time-constant (RxC) for a lower capacitance below 1µF so we can use film-capacitors. If this is not possible it is important that there is at least 1 Volt DC over the electrolyte and that a good quality capacitor is used (BHC Aerovox, Nichicon, Epcos, Panasonic ...).

Bipolar capacitors should be avoided, they are in fact two electrolytes in series with opposite polarity and twice the capacitance. This is against the "Less is more" High-end philosophy. In many cases the polarisation can be calculated or measured.

This is often a problem in Opamp-circuits with a symmetric power-supply where only the (low) offset-voltage is available for polarisation. In this case a lower capacitance is often possible because of the high input-resistance of the opamp and we can use film-capacitors. Or we can replace the capacitor with a wire-link.

By choosing the best solution for every application the best sound-quality can be reached. And investing in quality capacitors will have a more positive effect on the sound quality than with any other component.

Comment by Joost

Within the company we had some discussion about Tantalum and other polar capacitors.

I found two very good documents on this subject. One by C. Bateman and one by W. Jung. Both documents are in the download section of our website (Theoretical and practical Electronics papers).

Although the documents are “old”, I did not find any recent quality documents (comments welcome!). Most opinions on this subject are based on those two studies.

In a recent article in the Dutch magazine Elektor, they measured an Electrolyte capacitor intended for the input stage of their Op Amp based power amplifier (yes they did! 32 NE5532 per channel). They rejected it because of the distortion compared to a film capacitor. The distortion can be reduced by connecting two capacitors back to back, as Jung and Bateman also found.

Bateman find high levels of distortion in Tantalum capacitors and rejected them for audio usage. Jung also did notice the high distortion, but with some special techniques they can perform reasonable. Will be continued.


Interesting links about capacitors:

Quad 66 Revision Kit Design considerations

We get more and more questions about how the sound quality of the Quad 66 can be improved.

The 66 is often used together with the 606 which may be one of the best Quad amplifiers ever made. But there certainly is room for improving the 66 by replacing some of the components with better quality.
  • The bipolar capacitors can be replaced with Nichicon polar electrolytes
  • Some TL072 opamps can be replaced by Burr Brown OPA2604
  • The switch-IC's van be replaced by better quality with lower THD

It is not clear why Quad used cheap bipolar 100µF electrolytes between the stages. We switched on the 66 and measured the voltage over each electrolyte to determine the polarity. This way we could replace them all with high-quality polarised Nichicons which are a lot better for audio. Bipolar caps are in fact two capacitors of twice the capacitance in series and this doesn't correspont with the "less is more" High-end Hifi philosophy.

The TL072 can be easily replaced with the pin-compatible Burr Brown OPA2604 which offers a much better sound. On the Disc-board we will use OPA2134 because of it's good signal/noise figures and excellent behaviour at low signal levels.

The TL074 quadruple opamps are still a problem as there is no Burr Brown alternative in a DIP-package. We are looking into the possibility to replace them with OPA4134 with a SOIC to DIP adaptor but maybe we will leave them in place.

Finally we are testing Fairchild and Maxim alternatives for the switching IC's

The first 66 we revised had a much more transparent and dynamic sound after replacing the critical components. We are still making decisions about some other components and writing the revision manual and we will offer the kit in the Webshop in the beginning of 2011. We can already do the revision for you in our workshop.

See also the forum-discussion Quad 66 Hybrid Elements on this subject.